12. Percentages again: will people ever learn?

A serious news item ruined by shoddy reporting.

People, even BBC journalists can't calculate percentages to save their lives! What is so difficult?

Drug-Driving arrests 'on the increase' - government

This article claims that drug-driving arrests were up 800% "from 70 to 530 over a year. Putting aside the statistical fluctuations in these numbers, the increase is 530 - 70 = 460, which is a fractional increase of 460/70 = 657%, or 650 in round figures somewhat less than the 800% figure quoted. Why can't journalists do simple calculations that would make a teenager blush? This is not the first time I have commented on this.....see my blog item number 3. It's almost as if there is a magic number around which percentages (calculated by the ignorant) suddenly go up by 100. For example, starting from 100 to be easy, a 10% increase would take us to 110, a 100% increase takes us to 200. Probably around about 300 people seem to forget to take off the initial 100, and 300 suddenly becomes a 300% increase. Perhaps 290 would still have been a 190% increase and 299 a 199% increase (correct), but who knows? That is why it seems that you always have to go back to the raw data as percentage calculations are nearly always wrong. Unfortunately, sometimes only the percentages are given and you are left wondering what it all means.

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