I have essentially a propagation-of-error problem I run into frequently with my scientific data. For example, I have three samples, each of which I take two measurements of. So, for each sample, I can calculate a mean and a standard deviation. However, I can then calculate the mean of the three samples together, and a standard deviation for this mean. However, this feels like it underestimates the deviation, as we have not factored in the uncertainty in the mean of each. To be specific with an example:
I have three samples (which are supposedly identical), called A, B, and C. Each sample is measured twice: for instance, A is 1.10 and 1.15, B is 1.02 and 1.05, and C is 1.11 and 1.09. Using Excel, I quickly calculate means and standard deviations for each (A: mean 1.125, stdev 0.0353...; B: mean 1.035, stdev 0.0212; C: mean 1.10, stdev 0.0141). But then I want to know the mean and standard deviation of the total. The mean is easy: 1.09; I can also calculate the standard deviation for that calculation: 0.05. But this seems to not take into account the error found in the numbers I am averaging.