Estimating the standard deviation or variance

4.0	***	14.2	14.5	***	17.6	***	18.6	20.8	21.4
22.1	22.2	***	23.2	27.0	28.0	29.6	***	30.9	31.0

Consider the sample of n = 20 scores. Notice that they are listed in order from 4.0 to 31.0, but that some of the scores are obscured by ***. The mean, M, of these scores is 21.0.

Suppose you are asked for the sample standard deviation of these scores. Since you don't have all of them, you cannot calculate it. Which of the following is the best estimate of the sample standard deviation?

16.1

27.0

7.6

0.4