Discussion

Each person who applies for an assembly job at Robert's Electronics is given a mechanical aptitude test. One part of the test involves assembling a plug-in unit based on numbered instructions. A sample of the length of time it took 42 persons to assemble the unit was organized into the following frequency distribution.

Length of Time (in minutes)	Number
1 up to 4	4
4 up to 7	8
7 up to 10	14
10 up to 13	9
13 up to 16	5
16 up to 19	2

What is the standard deviation (in minutes)?