I need to work out the standard dev for the following table
Crushing Strength N/mm2252831343740Frequency (f)153042362418
However I have used a spreadsheet to calculate the answer and I'm coming up with 10.46422477. This is also the figure I get when working it out on the calculator.
The problem is the mean I worked manually using formula: (sigma) crushing strength x frequency / (sigma) frequency, so 5349/165 comes to 32.41818182. But the mean using excel comes to 27.5.
Can someone work out the standard deviation for me so I can see where I'm going wrong?