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.
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?