# Can you calculate standard deviation with different variables?

I have a set of data that are basically used to plot points on a graph. When calculating standard deviation I have only ever calculated it with one variable. Is it possible to do it when there's a variable affecting the value you want to calculate the standard deviation for? I have length and 1/Root(C) which we can call X for now: example of the data:

length:x
0.1:0.139
0.2:0.288
0.6:1.052


Could I multiply each number by a value that makes there length = to the largest length (0.6) then use the mean of the values example

0.1:0.139


becomes

0.6:0.834

• I don't think this is at all clear. What data are you trying to compute the standard deviation of? What is $C$ and what has it got to do with anything? – lulu Mar 29 '17 at 20:30
• @lulu C is count rate. Im just asking can I combine the two data items together because using different lengths is surely wrong – Holly maine Mar 29 '17 at 20:33
• What two data items? Length of what? I think you assume your reader is aware of whatever you are working on....can you describe the problem clearly from the beginning? – lulu Mar 29 '17 at 20:35
• @lulu its data i got from doing a physics practical to prove the gamma ray inverse square law. Length is the distance the source is from the counter and C is the count rate – Holly maine Mar 29 '17 at 20:39
• Do you have a model of the data? Something you are trying to fit to the data? If so, you can hope to find optimal parameters for it. It's easy to compute the standard deviation of any data you specify...but absent a model, it's really not clear what it is you are asking for. – lulu Mar 29 '17 at 20:44