Tuesday, February 08, 2005

Twenty per cent reduction

Why have so many journalists made the same mathematical error. Today the guilty party is the London Independent's David McKittrick, but I feel bad singling him out seeing as SO MANY Irish journalists have made the same basic error.

I can't remember if any article on the new speed limits actually got this right. According to McKittrick:
The most significant change is on rural regional and local roads, which the Irish often refer to as non-national roads, dropping from 60 mph to 80 km/h, the equivalent of 50 mph. This is a reduction of 20%.
There is no need for that last sentence. Anyone can understand what a 10MPH reduction means, but almost every paper has thrown in that last little item on the percentage reduction. If only they had calculated the percentage reduction correctly.

If you start with 60 and take away 10 that is a reduction of 1/6 or 16.7%, not 20%. A 20% reduction would make the new speed limit 48MPH. Why have so many made this same mistake? Are these journalists being ripped off every time they go to a sale offering 20 or 30% off?

{The real percentage should be 17.15% because 50MPH is actually slightly greater than 80KPH, but that's not the big issue.}