Saturday, 30 April 2011

The devil is inaccurate


Charles Williams always insisted on what he termed accuracy - a trait essential to an editor of the Oxford University Press; but more than this, C.W. regarded inaccuracy as a sin: characteristic of evil.

And he was right!


What is accuracy? the main components are validity and precision.

Validity mean that a measurement is truly representative of what it claims to measure.

Precision refers to the statistical exactitude of a measurement.

So, if we were measuring the average height of the adult English population it might be valid but not accurate if the sample was 1000 randomly chosen subjects (because a random sample is representative of the whole population), but if the scale was only segmented in metre lengths, the estimate would not be precise - because the measure would only be to the nearest metre.

A sample using a one millimetre scale applied to a non-random sample (e.g. the first 1000 people you found in the telephone directory or met in the street, or 1000 women but no men) would be 1000 times more precise (because measured in millimtres not metres) but would not be valid, because the sample would not be representative of the English population.


As a professional epidemiologist I fought a constant, losing, battle to emphaisize the greater importance of validity than precision.

e.g. and references.

It is more accurate to have imprecise but valid knowledge than precise but non-valid knowledge - yet precisely measured garbage is the material of modern science, administration and politics.


This applies to everything in life - it is always infinitely better to be approximately right than precisely wrong.

The sin of inaccuracy is in claiming or assuming that precision somehow compensates for invalidity, or that greater precision somehow renders validity irrelevant.

This sin is endemic in modern administration - large, complex, quantitative databases are regarded as both essential and sufficient for policy - despite that the information in such databases is always invalid.

Always invalid because the process of data collection is not-even-trying to be valid - the data collection is indeed part of the policy, designed to support policy and not trying to understand the world.


I once termed this system Infostat -


So accuracy properly implies maximum validity as an iron rule, and precision only as an optional aspiration.

And this is not a technical, methodological point: it is a moral imperative.



Alex said...

Invalid claims may be due to ignorance of what constitutes validity in a particular case. Imprecise information may result from inadvertency or from an unsound, but not necessarily sinister, means of gathering it.

If culpable ignorance or the intention to deceive can be established, perhaps then and only then, is it reasonable to describe an inaccuracy as sinful.

One sin that's endemic in modern administration is the deliberate intention to mislead by representing ideological purposes as objective data or vice versa.

Bruce Charlton said...

" Imprecise information may result from inadvertency or from an unsound, but not necessarily sinister, means of gathering it."

Well, yes - but why are incompetent people gathering data, analyzing it, publishing inferences?

The short answer is professionalization and careerism.

For example in 'medical research' more than 95 percent of the people are incompetent because they do not know anything about medicine - nor do they care about their ignorance.

They do it because they are paid, and to get the status of pretending to improve health.

This kind of ignorance is culpable.

The Crow said...

As important, or more so, than any form of measurement, is honesty.
Without it, nothing else is valid.

Alex said...

Depending on the circumstances, incompetence is undesirable but is it always sinful?

I was not aware that medical research is sometimes conducted by people who are culpably ignorant. I suspect, of course, that some 'researchers' are more or less expected to 'find' therapeutic answers that will be advantageous to the manufacturers of drugs and such like.

On the pretence of improving health, here's a case in point. GPs will now importune patients whom they consider should be persuaded to take statins. I don't know how effective these drugs are in lowering cholesterol; but if doctors have a cash incentive to prescribe them wholesale, this is bad medicine, I think.

Bruce Charlton said...

While there may be individual exceptions, I believe that statins do greatly more harm than good; they make many people feel worse, and cause actual damage in some (I, for example, had my vision permanently damaged by statins when I was foolish enough to believe the hype) - indeed the whole 'lipid hypothesis' of heart disease is nonsense and always has been.

Probably, the epidemic of heart disease of the middle 20th century was caused by some unidentified infectious agent - like most epidemics.

For more evidence you could try doing some relevant word searches on Dennis Mangan's blog -