A computer simulation program is used to produce 150 samples of a random variable. The

samples are grouped into fifteen batches of ten samples each. The batch sample means are

listed below:

0.228, -1.941, 0.141, 1.979, -0.224, 0.501, -5.907, -1.367, -1.615, -1.013, -0.397, -3.360,

-3.330, -0.033, -0.976.

The sample standard deviation is . (Use two digits to the right of the decimal point.)