Tuesday, 5 May 2015

Who to vote for? Some of the above

As Bristol's premier data science organisation —and one of the few press outlets people trust— people ask us: who should we vote for?

Our answer: anyone you like —but.

Which is where our game theory work comes into play.

First though, consider all the members of Clifton Tank Command who kept on saying "but the nurses!" and "but the schoolteachers!", when arguing against the RPZ.

  1. It's probably the hospital porters who have the worst job. Ask them how little they get paid for a 10-12 hour shift? It's just that "but the hospital porters!" doesn't conjure up those Florence Nightingale scenes of the selfless nurse —so doesn't get used when seeking sympathy.
  2. Ask the nurses, hospital porters or other staff what is the worse thing to happen to them in the last five years. The Clifton RPZ or Jeremy Hunt's "reforms"? Because it won't be parking (probably).
  3. Similarly, ask the school teachers: "what is worse, the Clifton RPZ or Michael Gove's 4+ years in charge of education"? Because it won't be parking (probably).
All those people who were citing teachers and nurses in opposition to the RPZ now have a choice: do they vote for the parties that made things worse at a national level, or do they not. Because if they do vote Tory or LibDem, they abdicate the right to pretend to care about the wellbeing of all public sector employees.

What about the rest of us? This is where it gets interesting, especially in Bristol West.

  1. In 2010, the fact that the Conservative Party could gain an outright majority by working with the LibDems meant that they needed them, either in a coalition or on a vote-by-vote basis. 
  2. The LD team went for coalition. This gave them a seat at the table —but in exchange, they gave the Cameron government the majority they desired. 
  3. In exchange for that majority, the LDs got support for two referendums: electoral reform and house of lords reform. They got promised them, but then had the Tory party actively opposing them, and plans fell through. If electoral reform had gone through, all the smaller parties would be happy this election, and the LD would stand a chance of counting MPs in double digits next week. As it is: they lost. And in doing so, lost all that they could have gained from the coalition.  They've made their play —and lost.
  4. Theymade the mistake of promising support for five years. Once they'd done that: all negotiating power was lost.
  5. This week, they are arguing that they can be good for either a Labour or Tory government -but to negotiate hard with either party, they need to be absolutely prepared to work with the other party. Which means if you vote for them, you, the voter, don't get any say in who is the government. Only, possibly, the identity of some of the members of that government, and one or two of their actions.
If the LD are to have power next time round, they need to play differently. First: don't trust Cameron to deliver on any promise. You want agreements -get a lawyer to spell out all the T&Cs, not some handshake over tea. Better yet: supply votes on a case-by-case basis, and demand concessions on every single vote. That way lies power.

They're slowly exiting the Blair era, but following the press too much -making promises about "never forming a government with the SNP" which actually hampers their negotiating options. It would have been better to be vague. 

Just move to North Somerset now. We don't need your folk in the city.  

It was only the early 1990s when Bristol West was conservative, but nowadays that's viewed as utterly unrealistic —which is why they can stand up candidates that are quite happy to denounce the Cycling City work as a waste of money, whilst not denouncing the Managed M4 as equally useless for the majority of Bristol W commuters. She's also stopped updating her web site some time in February, showing her commitment to getting elected or an understanding of computers on a par with UKIP candidates.

One thing that is notable is that their agenda has moved from anything forward looking, even from anything about preserving their accomplishments (where is that Michael Gove person? Or Jeremy Hunt?). Instead they've actually focused on being anti-scottish. And while they say "no, anti-SNP": that's not how it comes across. It comes across as saying Scotland isn't welcome in shaping what kind of nation Britain will be in the 21st century. Which a fair Scots find somewhat offensive. A majority of the country did vote to stay in the UK, so why push them away?

They've moved on from banners saying "Bannockburn, 1314 -we remember". Glasgow has embraced them —and in doing so, they've embraced Glasgow; Scotland's city of the workers, with the history of Red Clydeside and the closest Britain has ever come to a communist uprising. They promise to give Labour the heritage they've forgotten.

Here's some fun.  The Green party appear to be in second place in Bristol West, not that far off from Labour. And they do seem to have some more posters up than the others —though given the general lack of posters, that's fairly meaningless.

Bristol W. is the Green's targeted second seat? Does that mean they will win it? They presumably hope that by repeating it often enough people will believe them and they'll get that majority. At the same time, there's that risk that the anti-coalition vote will be split, Stephen Williams will get in, so giving the LDs more negotiating power, and so the likelihood that Cameron stays in his office.

Independents for Bristol

They exist, apparently. Maybe as local councillors they'll have a role.

Plaid Cymru

They never come over to Bristol to campaign. And there's us with a River Avon. Someone should stand on their behalf. Over in Wales, it'll be interesting to see what happens, and if they can gain that same momentum that the SNP have got.

Emigrate to Spain and spend the rest of your miserable life whining about immigration. For reference, the people in Edinburgh didn't attack Farage out of racist hate of the English: they did it because he's a pillock.

Friday AM will have Labour and Tory short of a majority, 2+ smaller parties trying to have power and influence over them by promising support. The LD experience of 2010 has shown the danger of a coalition with the conservative; Scotland learned about the worthlessness of Cameron's promises the day after the referendum, so won't be sitting down with him.

Tory will be able to talk to: DUP, LD and perhaps UKIP. Hopefully UKIP will be irrelevant. DUP aren't too different from the conservative party, and won't make things better or worse. The LDs? Will they have learned their lessons from last time and negotiate better, or again, give up their ideals for an office with a phone?

Labour is going to have to talk to the SNP, which is why Milliband's absolute refusals to work with them are shortsighted.

Meanwhile, the majority of the press will be saying a government with the SNP in it —or supported by the SNP— is not legit. Well, here's some bad news, since the mid 1980s Scotland has been almost entirely unrepresented by any conservative government (thank the "campaign for a tory-free Scotland" there). That's led to a pretty abusive relationship coming up from the south, the Poll Tax (seen above) being the key example. Having a government with the SNP involved would actually be fairer than those conservative governments from 1988 to 1997: Thatcher and Major.

Anyway, your call. Just bear in mind that if you are voting LD in the general election, there is a high chance you are actually voting for a conservative government. Voting Green you may be making Bristol West stand out as a green city, or losing your choice to have a say in the country. 

Us? A joint SNP+Labour government with a couple of green MPs would be an interesting government to have.

PS: what about their actual manifestos? Meaningless. Why analyse things made up for press releases. Interesting that only the SNP think Trident is an utter waste of money; only the LibDems have raised the fact that our government's monitoring of everyone's emails may be something to question. That topic didn't even make the press. But then neither has the environment. Arguing about whether Scottish MPs could form part of a government take priority over policy, apparently.

Sunday, 3 May 2015

The Zone map that Clifton Tank Command Dare Not Show

Of all the maps that appear in the RPZ tank-battles, this is one that does not surface.

Its the map of where the air pollution levels in Bristol exceed the levels that are considered safe.

Look at that map. What stands out (to any resident of the city)

  1. The entire city centre is an Air Quality Management Area. 
  2. Clifton manages to dodge it, primarily by being above the town -though as it gets into Kingsdown and Windmill hill, height is not enough for the roads to stay breathable.
  3. The roads into and out of the city are pollution hotspots. That includes Gloucester Road, Bristol's "most popular" cycle road.
  4. The Frome valley pollution zone tracks the M32 perfectly.
We can't split the pollution into resident, business, public transport and commuter, apart from the Rupert Street bus & taxi only road —the one with the worst pollution in the city. Usually.

What we do know from the queues of cars on the A370 and A4 Portway every morning is the number of people who drive in to the city.  Any morning you can walk onto the Suspension Bridge and look down at the queue of cars who have done the portishead-M5-A4 route (and from other places, including Clevedon & Weston) and are now stuck in the Avon Gorge, fuming at the empty lane next to them along which park and ride buses whizz past. Any morning you can go to the footbridge above the M32 and look down at the line of near-stationary cars, all sitting with their engines on.

And in any inner-city area that is not an RPZ, you can watch the cars go round and round in circles looking for somewhere to park.

And now what -we have a  Somerset MP actually surfacing in the county to complain that Bristol's RPZ has had knock-on effects for Leigh Woods. Well, that's unfortunate —but not a reason for Bristol to attempt to do something about their air-quality. And an RPZ, if it actually helps alternate transport options in the area -including N. Somerset- will.

What's not covered here is that Leigh Woods has always experienced commuter parking -which was getting worse with the cost of crossing the Suspension Bridge, even before the RPZ went live. People who lived in the hinterlands weren't prepared to pay £2/day to drive over, and leigh woods became the cutoff point.

Well, Leigh Woods is free to roll out an RPZ too. As is Long Ashton. We can't say "but the roll-on effects" should stop any attempt at trying to make the city better to live and breathe in.

As for the residents of Portishead who say that Bristol is now trying to control where they work? 

Sorry. We are trying to control how people get to to work, to adopt options that aren't so literally poisonous to the city.

And the people who say "hold off until there's a viable alternative?" The residents of Portishead were all happy when the council spent £3M widening a roundabout, to reduce the time they spent queuing to get onto the M5 and then to work in Bristol or the North Fringe. £3M for what: one roundabout? Which, in a manner obvious to those of us who actually understand Queue Theory (it's not rocket science, you know), does nothing except move the traffic jams slightly closer to the city. North Somerset, under the guidance of Elf-King App Rees have spectacularly failed to get the Portishead Railway reopened for passenger traffic. They've actively opposed cycle facilities along their roads,  and actively campaigned against cycle routes through their two-cars-per-household-mock-villages.

It is the repeated choices and actions of the residents of North Somerset that have failed to provide that viable alternative to driving.  Why should Bristol care about those decisions? By leaving the city, these people abdicated their right to influence the decisions the city makes. And, in the hands of their democratically elected council, held back any form of progress. When they do attempt something, it fails so badly it gets ridiculed on national TV.

North Somerset are the hinterland of Bristol, not just geographically, but culturally.

Which is why we in the city can't afford to be held back by them.

We aren't trying to tell them where to work. We aren't even telling them how to commute.

What we are doing is saying "The road space and air quality in Bristol is too precious to waste on free commuter parking." By taking that away, anyone who wants to drive in still can —except they now get to pay for that right, so making the external costs of commuting in what is usually a single-occupancy vehicle tangible. In doing so it makes the now-internalized cost of driving in closer to that of using public transport, including the Avonmouth P&R site. It may even provide motivation for the residents of Portishead to push their councillors to get their thumbs out their arses and start working on this —maybe even setting up a Metropolitan Transport Authority covering the CUBA district, so actually giving them some input on Bristol's traffic plans. And, given there's an election coming up, maybe talk to their candiate MPs and say "will you do something for transport in the area other than staged photos in Leigh Woods?"

What we can't do is stop the RPZs and say "business as usual". Because its not just that everyone driving is causing congestion, they are helping poison the city.

Next time someone talks about resident parking zones, say "what about the air quality zones?". And if they don't have an answer, instead complaining how they have to use P&R instead of queueing to get into the city —you don't have to feel sorry for them at all.

Friday, 3 April 2015

MTB trail: "The Corrie Road"

We've not done much coverage of the mountain bike trails in the area for a while, trails such as Nova, Super Nova of Ashton Court, along with Yer Tiz in Leigh Woods.

To catch up then, lets look at another council-manufactured technical trail through the trees, "Corrie Road".

This trail starts at the local village of "Beddmynster-upon-Avon", known locally as "Bemmy". It can be reached by train from Bedminster Station, or directly from Templemeads via the new Clarence Road segregated path. There also is parking by the local store, "Asda". They sell food and fresh bread there —visit it if you haven't already! Note that while this shop is surviving, the adjacent library is at risk of being shut down and converted to gentrified housing.

As well as food from the local store, you can also buy bicycle spares at that eclectic Bristol chain, Motorman. You can also get waterproofs, footwear and other things from the friendly folk at Taunton Leisure. For anyone planning to make a weekend of it, there is some accommodation right in front of the trail itself. (warning: some of those reviews are a bit negative)

The trail itself is an all weather off-road surface. There are a number of tree obstacles to "work"; making for a very swoopy course. It is probably the easiest to do as a nighttime-excursion, primarily because of the streetlights placed in the middle of the trail itself! You won't be needing any helmet lights here!

It's also flat and straight: no real navigational problems. We'd recommend it as an intro to MTB-ing for children, were it not for the fact that if they get an obstacle wrong and fall left, they will end up going under the wheels of an HGV —this road is notable in that it is still a 30 mph road; the 20 mph zone begins behind it.

Here then is the trail done at near-race speed, though the rider doesn't clear every slalom gate correctly.

Sadly, the trail does peter about rather abruptly, leaving you with a choice of where to go next. Turn right to get to The Chocolate Path, where you can head to the Avon Gorge, the pump trail, or Ashton Court itself via the "metrobus wasteland" —formerly known as Festival Way. Alternative options are head back towards the starting point of the road via the Chocolate path and a non-technical pavement route, or simply return the way you came. Yes, The Corrie Road path is two way —do watch out for oncoming cyclists as you commit into the bend around a tree.

After a ride, we recommend the pub "The Windmill" in Bedminster, or the Nova Scotia at the end of the Chocolate Path—both of which are fine establishments to consume a beer or three.

Wednesday, 1 April 2015

Our letter to @BristolPost

We've just sent a letter to the evening post. We encourage others to do likewise. This was not sent to the letters@ address -it was sent to the new desk. Its in their part of the web site in which the article is printed, and we want action, not some letter a few days from now.

To: epnews@bepp.co.uk
Subject: April 1st "cyclists and lorries" article


Some people view our Bristol Traffic site as some kind of spoof.

Our contributors have twice encountered HGV/cyclists collisions in Bristol. One of our contributors lives in Bath, not far from Lansdown Hill, where a four year old girl and others were killed by an out of control HGV. When we cover such things, all notion of humor is gone. We treat the incidents as the awful events they are.

Yet your paper thinks this topic merits an April 1st story.

That is probably one of the most tasteless and insensitive stories you have ever printed.

Here is our response.

As we say at bottom of our article: it's not to too late to your article offline and replace it with an apology, one you can reprint tomorrow.

We even grant you the right to use any of the photographs in the above article with such an apology.

Please let us know what your plans will be


the Bristol Traffic team.

Evening Post: not funny at all

The Bristol Post has an April 1st story, Cyclists to escort all lorries through Bristol city centre in bid to reduce speed and improve safety. someone thought that was funny.

More precisely, someone thought that was funny less than eight weeks after a tipper truck killed four people in Bath —something local and awful enough that the BEP felt worth covering themselves. A truck crash in which the driver and the company chairmain have been charged for Manslaughter on. Think about that: if the police not only charge the driver with something worse than "Causing death by careless driving", but even the company owner with manslaughter, there must be evidence of a set of wilful decisions resulting in what even the police consider to be killings. Not accidents: killings.

What those decisions are, we are yet to know. Overloading the vehicle? Maybe. Driving with defective brakes? Quite possibly. Driving down a road with 6.6" signs at the entrance and a use low gear sign later on? Again, highly likely. Some papers have been claiming the width sign had been knocked down -but there are two of them, which kind of makes that statement doubtful. A wilful decision to take a shortcut seems suspiciously likely.

Yet the BEP thinks lorry safety is something worth having a laugh about.

Whoever wrote that article didn't look through their archives for keywords like "HGV and life-changing injuries"

They've certainly never heard the scream someone makes as an HGV drives over them.

On a bicycle, what's likely to kill you is an HGV.

And the Evening Post thinks this is funny enough to make a joke out of on April 1.

Anyone who rides a bike and actually pays for the evening post should reconsider their decision from now on

Anyone who looks at the web site should immediately download the Adblock plugin (Firefox, Chrome) so the evening post doesn't get that advert placement revenue.

As for the Bristol Traffic site? After this post we refuse to place any links to Bristol Post articles. Ever.

To the evening post, we want you to recognise that this isn't funny. Anyone cycling around the city fears the HGV, fears it coming up behind you at an ASL. Or even worse: a roundabout. If you've ever felt your bike being nudged forward by an HGV at a roundabout as the driver looks to their right, you'll know a moment of cool terror. Moving HGVs are just bad. You fears them passing, as you look to the side to see if its indicator lights are on, warning you that it's about to turn over you. And that's if you are lucky, if they do indicate.

Please pull the article and post an apology for such a tasteless story.


As for the author for the article: we extend the offer of a bike ride round central Bristol. The old BEP offices to templemeads should be enough to convince them never to think such an article would be funny. If that's not enough, we'll take them to Bedminster and then down Coronation Road, returning by Anchor Road to the centre, up to the Bearpit roundabout and then Newfoundland Road. If they aren't left a gibbering wreck vowing never to get on a bicycle again, maybe they will write prose so awful in future

Saturday, 28 March 2015

Tribars: not for urban use

Following on from our coverage of the fact that its sporty cars that lose from the 20 mph zone, we should look at the impact of the figures on bicycles.

Before anyone points out that bicycles are exempt from the limit, consider that if it becomes a speed that people expect, then going above it would create problems for people assuming you are heading more slowly. Though as any tax-dodger will point out, most people passing your or pulling out assume you are near-stationary and plan their manoeuvre  accordingly.

Even so, fast bikes don't have a place in the city. Stick to 20 or less: and if you can do that on the uphills you've earned those numbers.

What it does mean is that just like fast cars, tribars aren't needed in the city, nor are carbon wheels or polka-dot socks.

Yet here we see someone doing 6-8 mph in exactly that setup.

And while it's nice to see that they are staying way below that 20 mph limit, they do have their arms on the tribars. Which is another way of saying "their hands are a long way from the brakes".

As they reach the end of Kensington Place, they approach the give way point at Lansdown Road, where this lack of braking ability almost catches up with them. Because there's a Range-Rover heading north from Lansdown Place —below that 20 mph— and the cyclist needs to make a bit of an emergency swerve from going into it. Which could have damaged the paintwork on the car as well as written off some carbon wheels.

Bikesnob is always taking the piss of triathletes. It doesn't really apply here, but it does send a message to all: plan ahead, keep your hands near the brakes especially as you approach junctions where you are expected to give way.

Note the yellow lines on the road. This is the Clifton Village RPZ, as you can see on a weekday it is now a wasteland.

Friday, 27 March 2015

Bristol's 20 mph zones: it's the hot cars that lose

The 20 mph zone has been up for a year now, so it's time to review it as drivers
  1. It doesn't make things slower. Really. It's the delays at junctions & in queues that increase journey time.
  2. It makes things calmer. There's less pressure to put your foot down when you do clear a junction.
  3. Similarly, as you approach a junction, you can coast down more gently. 
  4. As you are going a bit slower, you can take time to look around, which gives you a better view of pedestrians. This is tangibly better at night.
  5. It actually helps you pulling out from side roads to main roads. Why? As everyone is moving slower, the time window for you to do things like pull a right turn with cars approaching from both directions is larger.
  6. Fuel economy? No obvious difference. The engine may be less efficient in 3rd than 4th, but you don't have to accelerate so hard, and can coast down. Of course, anyone in a hybrid car is laughing as their petrol engine can work even less, and gain more regenerative braking from the gentle slow down.
  7. Keeping track of your speed? Third gear low RPM seems to work. If you feel the need to go to 4th, you are going too fast. 
  8. Does everyone follow 20 mph? 22-25 is more realistic daytime speed; at night the speed goes up to 30 until the minicabs come out, when it ramps up 40 mph just when the drunk people start walking home.
  9. Increased road rage? No obvious difference. As the average commute speed is < 20 mph in Bristol (TomTom's unverified data, not ours), it's hard to see how it could be made worse.
  10. Collapse of businesses due to increased white van journey time. Not obvious. Congestion is the limiting factor, not maximum speed.
  11. Passing Bicycles? No harder or easier. It's still irritating to be behind someone going along at 12 mph. But the speed limit doesn't make passing harder. We just want fitter cyclists out there.
  12. Then there's the "two shopping trolley" man wandering round the streets these days. We have no idea why he has two shopping trollies full of his entire belongings, but he does, he goes down the roads (not the pavements) at about 4 mph. Again, 20 mph doesn't make a difference.
So who loses? People who spent money on fast cars. You shell out all that cash for a nimble toy, for an extra digit or two at the end of your car brand logo —"i", "GT", etc—, tinted windows and some wheels that just scrape easily.  The key "performance" benefits are tighter suspension, and most of all the ability to accelerate better. 

Which is now utterly wasted, as most vehicle's 0-20 numbers are relatively similar. And pootling around at 20 mph means no need for suspension that lets you do 90 degree turns at 35 mph.

That's enough to drive you to road rage: not the fact that you are doing 20 mph, but the fact you spent a lot of money on your status toy and are doing 20 mph.  Even if you want to go faster, there'll be someone in front who doesn't, who appears to drive at exactly 20 mph on a fast ratrun road  like Ashley Down, Pembroke Road or Filton Ave. It's almost as if some people, on noticing an important person driving a high-end Audi or BMW SUV actually take their foot off the accelerator, dropping from 24 mph to 20. Which we consider unacceptable and strongly condemn anyone doing this.

For those people who have spent the money, they expect something in return.

Fortunately, every so often the opportunity arises. Here is one our instrumented tax-dodgers going down Nugent Hill, Cotham, using the bike contraflow to get to the Arley Hill evening traffic jam, and so on to Stokes Croft. As they join the Arley Hill queue, you can see a green light allowing some traffic to slowly get out to places more interesting.

And here you can see the BMW 3 series YF57KTE getting some return on investment. First they can come off the speed bump while accelerating (suspension), then put their foot down to catch up with the cars in front. Those cars in front who are going through on orange we note.  But as the BMW is now almost joined up with them, it can do the "part of the same group" gambit and carry on through on the red light.

That tactic has given them a bit more speed than the vehicles in front, forcing them to negotiate what is effectively a chicaned right turn fairly aggressively: that suspension at work again.

They then need to put their brakes on, not because of the speed limit but because they've caught up with the car in front.

There: 15 seconds of real driving, out of probably 30 minutes of suffering. Not much —but that's all a 20 mph zone offers, at least during the early evening commute.

Saturday, 14 March 2015

An introduction to surveys

Richard Payne of ITV has asked by way of our strategic code-sharing and data mining partners, Twitter, how to explain bias and self-selection in surveys.

This is a topic dear to our hearts for a number of reasons:
  1. We consider ourselves to Bristol's premier data-driven traffic analysis site.
  2. We recently conducted a survey on traffic issues for the city —a survey which has been completely ignored by the evening post, the BBC and ITV.
  3. We have just received an SERC grant for a new project to measure the weight of the city using a stopwatch and a trampoline —and plan to conduct our survey at the BRI next week.
Population: A collection "set" of things you want to measure values from.  Examples: the population of Bristol or all the residents of an area within Bristol.

Subset: Some or all entities within a subset. Example, some of the population of Bristol or some of the residents of an area within Bristol.

Proper subset: A subset of a set which is actually smaller than the original set. (fancy mathematical word: Cardinality). Examples: some but not all of the population of Bristol, or some but not all of the residents of an area within Bristol.

What is important here is that, by definition, a subset of a population must not contain any members outside that population. As examples, a subset of the population of Bristol must exclude people from North Somerset. Similarly, a subset of the residents of an area within Bristol must not contain anyone who does not live within that area.

In our survey we actually measured the origin of our self-selected sample to assess this. We could have just ignored them, but instead chose to include them in our answers on the basis that it was easier just to leave them in.

Data: Numbers. May be analysed by somebody with a statistical background to reach some meaningful conclusions. Without those mathematical skills you'll end up with something as useful as having a rabbit do your tax return.

Measurement: Using some form of scientific mechanism to come up with data about the things you measure. Examples: determining the weight of someone with a weighing scale. Determining the parking and driving habits of people by recording where they park or tracking where they drive.

Invalid Measurement: trying measure something by using the wrong tools, badly calibrated tools or reading the numbers off wrong. Example: determining the weight of people by having people jump onto a trampoline and using a stopwatch to time how long it takes for them to stop bouncing.

Poll: Asking people for their opinions. This is different from a survey in that it is assessing the beliefs of those people, rather than through measurement. Example: asking someone how much they think they weigh rather than putting them on a weighing scale. Asking people about parking and driving rather than actually recording or tracking them.

Leading questions. A sequence of questions which may, unintentionally or not, change the answers to follow-on questions. As example of leading questions, imagine the following sequence
  1. Are you aware that being overweight can lead to an increase in coronary heart disease and diabetes?
  2. Do you believe that overweight people should be billed by the NHS for medical care for weight-related conditions.
  3. Are you a fat bastard?
After the first two questions, nobody will say yes to question 3.

Census: Measuring or polling a Population. Examples: people whose weight you want to measure, or the residents of an area whose opinion on parking you want to known. A census of a population is the only way to come up with a value of the measurement or poll which can be considered 100% accurate in terms of sample set. Everything else is incomplete and therefore inaccurate to some degree.

Survey: Measuring or polling a proper subset of a population —with the goal being to extrapolate the results to the entire population. Examples: weighing only some of the people in Bristol to extrapolate the weight of everyone in the city, or polling some of the residents in part of the city to extrapolate to the opinions of all the residents of that area.

Sample: The proper subset of a population used in a survey. Examples: some but not all of the population of Bristol, or some but not all of the residents of an area within Bristol. Another term is Sample Set.

Defensible: Something which you can present to people who understand statistics without being laughed at.

Invalid Sample Set: A sample for a survey which cannot be used to extrapolate to the entire population. Examples:
  1. Including people from North Somerset in a survey to determine the average weight of the population of Bristol.
  2. Weighing only those Bristolians who have been referred to the BRI heart clinic in a survey to determine the average weight of the population of Bristol and using a trampoline and a stopwatch to do so.
  3. Using too small a survey set for the size of the total population. Example, weighing two people and attempting to reach a conclusion about the weight of the entire population of the city.
  4. Attempting to conduct an opinion poll of residents of part of the city without excluding non-residents of that region.
  5. Attempting to conduct an opinion poll of residents of the city within, say, residents parking zone, yet deliberately choosing to exclude parts of the area —such as, say, Kingsdown and the city centre.
  6. Excluding some of the population on the basis that they do not meet some criteria. Example: excluding anyone who doesn't own a car from any opinion poll on the topic of residents parking.
  7. Conducting an opinion poll of the residents of part of the city by only asking those people who have opinions on one specific outcome of the survey. Example: asking only people opposed to residents parking of their opinion on the topic. Conducting a survey by requiring participants to perform some action such as posting in their survey. The latter tends to something called self-selecting samples.

In our survey 32% of respondees declared they couldn't afford a car. These people don't have valid opinions on parking, nor on other parts of our own survey.

Statistical Outliers

These are a fun thing in experiments. Something way out of the expected. You can include these in your answer, though you can also try and work out how the outliers got in there and then discount them —this is especially useful if you are trying to make sure the survey reaches the conclusions you want it to.

Look at our question on the number of wing-mirrors replaced since the 20 mph rollout.

70% of the respondees claimed that they hadn't replaced any wingmirrors since the 20 mph zone. This was utterly unexpected, and, if used when trying to determine the average number of wingmirrors lost per resident per year, we get an arithmetic mean of 0.87 mirrors/year -less than one!

Yet can be explained if we include two other facts from our survey
  • the number of respondees who asserted that they lived outside the city: 54%
  • The number of respondees who asserted that they were too poor to own a car: 32%
As we are measuring the impact of 20 mph zones, we should be discounting those people from our analysis of this question:

Discounting non-car owners: 70-32 = 38. Therefore of respondees who owned car, only 38% of them got through the year without needing a new mirror.

Discounting non-residents. 54-38: -16! Which seems impossible, unless you consider that many of those non residents will have driven into a 20 mph zone, and so lost a mirror.

Once you discount the non-car owners and non-residents, we get the result we expected: since the 20 mph rollout, everyone in the 20 mph zone has lost one or more wing-mirrors, with the average number being 3. At 15-25 pounds a shot,  that wingmirror-tax is yet another tax on the hard-working motorist.

Causality and co-relatedness
Again, fascinating. Merely because two things appear correlated over time, doesn't mean that one causes another.

In this question Why has congestion got worse in Bristol over 25 years?, 17% said BT added an extra digit to all the phone numbers in the early 1990s. Some people may say "so what?", or even "the growth in Bristol's population caused BT to add more numbers; that same population growth increased the number of cars, hence the resultant congestion". We say something else: It was the adding of that digit which made it possible, in a pre-mobile-phone era, to move to the city. That 17% were right. And from this survey nobody can prove us wrong!

Invalid Survey
Any survey that can be considered invalid from a statistical perspective. Common causes are: invalid sample sets, leading questions, bad measurement, leading questions and bad-analysis, including confusing correlation for causation.

For some examples:
  1. Asserting facts about the average weight of Bristolians through an opinion poll with leading questions conducted at the BRI heart clinic of 4-5 people, without even excluding any attendees from North Somerset. That fails: invalid sample set and leading questions.
  2. Asserting facts about the entire population of Bristol's opinions on residents parking through an opinion poll with leading questions conducted against a self-selected sample set of some people who care about the subject.
  3. Getting your maths wrong when you add things up, divide the answers, etc.
  4. Misinterpretation of results. Reaching the wrong conclusions. If you want to reach a set of conclusions, you are less likely to question the sampling or analysis if the outcome agrees with your expectation. This is sometimes called confirmation bias
One classic way to bias a survey is simply to discount the "don't know" respondees and other non-participants. If you explicitly exclude these people from your survey —example by asking different people the question until you get one whose answer is interesting enough to write an article about, you've got an invalid subset of the population, hence the results cannot be extrapolated.

This problem of the don't-know answer is particularly bad in any self-selected survey because the members of the population who don't not hold opinions tend not to participate in it. Instead you get that subset of the population who hold opinions one way or the other.  It is also common for any survey which requires an action on behalf of the respondee, be it jump on a trampoline holding a stopwatch, or fill in a paper questionnaire and then post it.


As you can see, it is a lot easier to produce an invalid survey than a valid one. More subtly, its very easy to misinterpret a invalid survey for a valid one without knowledge of the sampling and measuring process, and knowledge of statistics.

For that reason, while surveys can provide some data about a subject, you can't consider the conclusions to be valid without knowing about the sampling, measuring and analysis —and any bias of the surveyors.

When you reviewing a survey, you should really query
  1. The population for which the survey is meant to be analysing
  2. The sampling process conducted in order to get a valid sample set
  3. How things were measured
  4. If it is some form of poll, the sequence and content of the questions.
  5. Outliers: what were they? were any discounted?
  6. What compensation have you made for non-participants?
  7. How do you defend your claim that this survey can be extrapolated to the population it was meant to.
Alongside invalid surveys, you have bad reporting of surveys. Often this where an invalid survey has been conducted yet the survey is reported as if it is actually represented as "fact" or representative of the entire population. This is a shame that reporters do accept surveys so unquestioningly -as if they did, they'd realise how often politicians use bad maths to make decisions. Or, in the case of Bristol, to generate controversy for the local press where otherwise there'd be nothing in the papers to talk about.

Further Reading

We hope readers found this introduction to surveys and censuses informative and timely. Please practice what have learned by using some of the terms introduced above in your everyday conversation —at least once per day. Example uses

  • "Please can I sample some of your chips"
  • "the causality relationship between eating chips and being overweight is not clear",
  • "your survey is utterly indefensible due to its painfully awful selection bias and leading questions  —your attempt to extrapolate it to any larger population hence so ridiculous you'd fail a GSCE if you sat one this week"

For anyone interested in learning more about this topic, here are some great online books on the topic