I am new to Standard Deviation.
I have taken a 2 small samples of Data and obtained the mean, and Standard Deviation.
I have copied and pasted a picture of my spreadsheet.
As you can see i have worked out the SD and mean of each group.
I have subsequenyly worked out a combined average, and then worked out the differences from combined average from each of the seperate averages.
I then calculated the combined SD using the following Excel formula.
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=SQRT((A10*((C13^2)+(C17^2)))+(E10*((G13^2)+(G17^2)))/(A10+E10))
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I cant get my head round how the SD of each sample is 0, yet the combined SD is 1.17. Can someone please explain this? Maybe i have calculated the combined SD incorrectly?
I think you're calculating things correctly.
Each sampling taken separately has a standard deviation of $0$ because every item is the same price in the sample. There is no variation.
But when you combine the samples into one, now you have a variation, so the standard deviation is greater than $0$.