Expert: Ahmed Salami - 10/17/2011

Question
Hi Ahmed, First I'd like to say your answers are always very lucid and accurate. My question is really sort of pet peeve. If you increase a number by 100%, you double it. You wrote down a formula that makes it clear how a percentage increase translates into a multiplication of a particular number. In common usage, people say, for instance, "the price of cheese increased 300%". I believe that they are thinking (as well as their audience) that the price of cheese has increase by a factor of 3 where in fact it has increased by a factor of 4 (if I understand your no doubt correct formula). How can we educate people to this error? I say we take to the streets!

Randy P.

Answer
Hi Randy,
Yes, its a very common problem. I guess its up to us to point it out to others whenever we can.
It might be better if people just said "the price of cheese has gone up to 300%" to mean that it has increased by a factor of 3. As another example, 'percentage points' are used instead of percentage to make it easy to quickly understand increases also.
You're right though and in the end everyone just needs to be educated.

Regards