For the last couple of weeks Tim Harford’s excellent BBC Radio 4 programme More or Less has examined what, in the UK at least, has become a new salary benchmark for the media – the prime minister’s salary (a previous programme checked out the common “Country x is 1½ times/twice the size of Wales”).

From this More or Less has constructed the PMI or Prime Ministerial Index.

It works like this.

The PM’s annual salary is £142,500, which is a PMI of 1.0.  Other salaries are rated against that.  So the president of the USA has, they say, a PMI of 2.0, or twice the British prime minister’s salary.

It’s all done with a light touch as befits a programme that successfully demystifies the use and abuse of statistics.

Some politicians use it in a slightly less benevolent way.

Both our previous Labour and current Coalition governments have contrasted unfavourably the number of senior public sector managers who earn more than the prime minister, or as Tim Harford might put it, have a PMI of >1.  The Coalition has published a list of all civil servants earning more than the PM.

The sub-text of much of the comment is that this is wrong.  No one should earn more than the PM.

But hang on.

There’s this significant difference.

Top civil servants are senior managers at the peak of probably a long career.  They almost universally these days have a university education, probably a professional qualification, definitely major in-service training and development and they’ve worked their way up through a highly structured and competitive promotion process (I don’t of course say they’re all perfect but that’s another issue).

Politicians (I don’t criticise them for it – democracy is by far the least bad system of government) are people who want to run the country according to their way of seeing things.  Their main qualification is that they persuaded enough citizens to agree with that view, at least on the day of an election.  They are, as the best of them hasten to say, public servants.

The two things are just chalk and cheese.

I don’t want to go back to Victorian times when MPs received no salary.  And I don’t want their salaries screwed down so that the job becomes unattractive to all except the most ascetic or zealous.

But let’s at least recognise that these are two entirely different classes of occupation and, beyond the realms of programmes demystifying statistics, drop the use of the PMI in serious debate as a measure of relative worth.