In a textbook, I've come across the following statement without proof:
In any group of data, most of the data are within 3 standard deviations of the mean.
This is equivalent to saying that $$P(m-3s,m+3s)>.5,$$ where m is mean, s is the (population) standard deviation of any finite set of data. Is this true and if so, can someone point me to a proof?