With a normal distribution of performance (the classic bell curve), a standard deviation is simply a more precise measure of how spread out the distribution is. (educationnext.org)
As with years of data, the more teachers there are, the more precise the measure of the average effect is. (carnegieknowledgenetwork.org)
I would be interested in a more precise measure for the bananas. (heynutritionlady.com)