# Percentage/zscore

#### aire51

##### New Member
What percentage of drivers drive over 72 mph?

I know I have to convert 72 into a Z-score. Im not sure if thats the answer or more to it?

#### lex logan

##### New Member
A z-score represents the number of standard deviations away from the mean something is. Z-scores are associated with known probabilities; you look up a z-score in a standard normal table or using software to get a probability. But you have to understand what probability the table or software is giving you; you may have to a bit of manipulation to get what you want. Example: Let's say male adults in the US average 70" in height, with a standard deviation of 2.5". What percentage of men are above 6'3" ? You take 75 minus 70 and divide by 2.5, getting a z-score of 2. Plug that into Excel: =NORM.S.DIST(2,TRUE) gives an answer of .97725. But that's the probability a value will be less than 2; what we want is the probability someone is more than two standard deviations above the mean, so 1-.97725 gives .02275.