A simple question. I know in theory, it is possible to calculate standard deviation for two numbers. I am wondering if it is plausible to do that. My objective is to compare two arbitrary time series data for the same phenomenon and plot mean and standard deviation as error bars for every time point. I know that you could compare the two time series by taking Pearson correlation and such, but I want to compare how much the absolute values were in agreement at every time point. Any insights will be appreciated.
Update: Thank you for the answers. Let us forget about the time series. It is an unnecessary complication. My question is more fundamental. I am doing a biological experiment to measure a biologically relevant quantity, say concentration of a chemical in my cells. Ideally, I would do 3 or 5 or some number of replicates of my experiment to get an estimate of mean and standard deviation. But due to time limitation, complexity of my experiment and costs involved, I can only do two replicates. Now, I end up with two estimates of concentration. No one questioned me when I took the mean of these two quantities. But people were uncomfortable when I calculated the standard deviation. I could understand their concern but I want to get more insights into why it is ok or not ok to take standard deviation in this case? If it is not ok, what are my options?