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Practice, difficulty 2 of 3

Estimate a standard deviation

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An IT service desk grouped the time to resolve 50 tickets:

Resolution time (hours)	Frequency
0 to under 4	6
4 to under 8	7
8 to under 12	12
12 to under 16	13
16 to under 20	9
20 to under 24	3
Total	50

Estimate the sample standard deviation of the resolution time, to 2 decimal places.