Y Safle

In my last “Mistakes We Make When Thinking” article, I raised the issue of anecdotal evidence being given more credence than data. An example of this is usually raised in the contentious issue of speeding (and what we should do about it). Driving is an issue on which most people seem to have an opinion, but how often is that opinion well formed?

By way of example I offer this part of a conversation I had with someone (MW). He began:

1) Most people break the speed limit This is self evident.

This is neither self evident, nor necessarily right. MW used the term “most” which I would understand to be approaching, but not quite attaining all. Now here are some figures:

“On non-urban roads, more than half the cars on motorways and dual carriageways travelled faster than the speed limit; 18 per cent were travelling in excess of 80 mph on motorways and 13 per cent on dual carriageways. “On urban roads with a 30 mph speed limit 65 per cent of cars exceeded that limit, 32 per cent travelling faster than 35 mph. On 40 mph roads 25 per cent of cars exceeded the limit, with 8 per cent exceeding 45 mph.” [Vehicle Speeds in Great Britain: 2001]

So only on urban roads with a 30mph limit will you find even a clear majority of people breaking the speed limits, and even then it appears that a clear majority only go over by a few miles per hour.

These figures were collected on clear unobstructed sections of road with no road “furniture” or other impediments to progressive travel, so they are indicative of real levels of speeding.

So it is neither self evident, nor even correct, to say most people break the speed limit.

I drive daily and in the past I have driven extensively - 60 to 80k pa. As someone who keeps to the speed limit or just under, I am aware of a considerable amount of traffic passing me.

Yes indeed, but consider how much traffic you pass also.

This is easiest to do on a motorway. Travel at the speed limit on the motorway and count how many vehicles pass you and how many you pass. You will probably be surprised to find the numbers are broadly similar. You are more aware of vehicles moving faster than you than those moving more slowly, but this is an illusion (the same illusion that makes you believe you are always in the slowest moving queue when stuck in traffic on the motorway).

So this anecdotal evidence is unreliable. The statistical surveys show a different picture.

People are wired up to give greater credence to anecdotal evidence than actual statistics. But that does not make the anecdotal evidence right. careful analysis often tells a different story.

This did not satisfy MW, who protested (against the data):

Yes, I know this kind of [visual anecdotal] evidence will not satisfy you, but frankly I don’t care. If you can’t accept the evidence of your own eyes, then fine, you believe what you want to.

I know I cannot accept the evidence of my own eyes quite often. My eyes tell me that the sun falls into the sea each night. They tell me that I am always stuck in the slowest moving traffic queue. They tell me that this keyboard is solid and that the universe is full of things. My eyes and my brain that processes the images coming from my eyes are easily fooled. Evidence for a proposition can come from many sources, but we don’t accept evidence from any source if it is uncorroborated by testimony from other sources.

So it is not me who will “believe what [I] want to”, it is you. You are choosing not to take into account the testimony of carefully crafted statistical analysis yielding verifiable data in a scientific manner, because you prefer to believe the limited and imperfect testimony of your eyes. So it is you who will “believe what you want to”.

Look around, stand on a street corner one day and watch the traffic.

Of course, I have never stood on a street corner and watched traffic in my life!

There was more, which I’ll post in another thread. The point here is that we become quite attached to our anecdotal evidence, even when it is contradicted by hard data. But don’t point the finger at MW on this point. Instead, consider what you believe based on anecdotal evidence only, that might be contradicted by actual data? We all have our blind spots. Let’s search them out, and avoid this mistake in our thinking.