The first four deviations from the mean in a sample of n = 5 reaction times were 0.4, 0.9, 1.0, and 1.3. what is the fifth deviation from the mean?

The sum of standard deviations from the mean of all the observations must be equal to zero. The sum of deviations for the first four points is (0.4+0.9+1.0+1.3)=3.6; so the fifth deviation from the mean should be -3.6 so that the total sum of standard deviation will be zero.