Suppose you have a data set that is normal and with a mean of 78 and a standard deviation of 9. Within what range would you expect to fine 95% of the data?
within in 2 standard deviations of of the mean
i.e. 78 +/- 2*(9)
http://faculty.colostate-pueblo.edu/paul.chacon/156sum04/EDA_68_95_997.html