Sara is investigating the variation in daily maximum gust, t kn, for Camborne in June and July 1987.

She used the large data set to select a sample of size 20 from the June and July data for 1987. Sara selected the first value using a random number from 1 to 4 and then selected every third value after that.

(a) State the sampling technique Sara used.

(b) From your knowledge of the large data set, explain why this process may not generate a sample of size 20.

The data Sara collected are summarised as follows n = 20 (sigma) t = 374 (sigma) t² = 7600

(c) Calculate the standard deviation.