The standard deviation is "the amount a typical score differs from the mean."
So estimate the mean and then estimate about how far out you have to go from there to capture about half the scores.
Since you are thinking about how far the scores are from the mean, it is useful to be able to quickly see how far each score is from the mean.
Compute the standard deviation of data: Use the applet that computes statistics and ask it for the standard deviation.
Useful to see why you DON'T really want to compute a standard deviation by hand!
Make comments or ask questions about the applets or the web pages