The healthy food brand Nature Valley got some flak for its wraparound ad for its granola bar in Metro last month.

The copy read “We wanted to increase deliciousness by 200 per cent. So we put two bars in each pack.”

By normal arithmetic, that’s a 100 per cent increase, not 200 per cent. Was this a deliberate error to get the brand more recognition? The mistake got a lot of coverage in the blogosphere, and even made it into the current issue of Private Eye (No 1296, p9).

Given the inability of journalists and copywriters to master percentages, I doubt  there was anything deliberate about it at all. It was just plumb dumb. But not uncommon.  

Take the much more upmarket Financial Times. In his column in the Money section on August 27, David Stevenson wrote an analysis of the impact of longevity on pensions contributions. His numbers came from pensions consultants Redington and showed that that to provide an annuity of £25,000 a year at age 65 if life expectancy at 65 is (as at present)) 17 years requires a contribution of £2,000 a year during the working lifetime.

If life expectancy increases to 23 years, the annual contribution needed is, according to Redington's calculations, £4,699. That’s a 235 per cent increase, Mr Stevenson says. No it’s not: it’s actually a 135 per cent increase. (The increase is £2,699, so the percentage increase is 2,699/2,000 X 100, or 135 per cent.)

It’s not an isolated slip – all the percentage increases in this article are wrongly calculated.  For example, the increase from £98,585 to £157,126 (the amount you would need to invest at the start of a working lifetime to provide an annuity of £25,000 at 65 for 17 years, or for 26 years) is given as 159 per cent. It’s actually 59 per cent (actual increase 58,541, divided by 98,585 X 100 = 59.38 per cent).

Personally I’ve always taken the view that percentage increases of over 100 per cent are so confusing for the reader than they are best avoided. Far clearer to say something has tripled than to say it's increased by 200 per cent.