Combining two means and SDs of one group I have the mean & SD of several different time points, I want to combine them into one mean and SD. Is it possible?
Example;
At 1 hour; mean=5, SD=1, n=42
At 2 hours; mean=6, SD=3, n=42 and so on.
Any equation to do that?
Note: at some time points, I have different group numbers (n), due to missing one or two patients.
I really appreciate your help, thanks!
 A: Rules to pool mean and variance of two groups can be found in O'Neill (2014) (Result 1).  The formulas are:
$$\begin{equation} \begin{aligned}
\bar{x}_\text{pooled} &= \frac{1}{n_1+n_2} \Bigg[ n_1 \bar{x}_1 + n_2 \bar{x}_2 \Bigg], \\[10pt]
s_\text{pooled}^2 &= \frac{1}{n_1+n_2-1} \Bigg[ (n_1-1) s_1^2 + (n_2-1) s_2^2 + \frac{n_1 n_2}{n_1+n_2} (\bar{x}_1 - \bar{x}_2)^2 \Bigg]. \\[10pt]
\end{aligned} \end{equation}$$
To pool a larger number of groups you can proceed recursively using these formulae.
A: Unless you truly believe there is a time difference-driven discrepancy, just pool them.  
Imagine you are measuring the heights of students in 12th grade.  Does it matter if one class comes at 1 PM and the next at 2 PM for measurement?  No.  You just deal with it as one sample.
On the other hand, if you are doing the measurements in Q1 (January) and Q2 (April), you should likely assume the kids grew.  If that is the case, it is a different problem - and a harder one.
More info on what you are trying to find would help.  But, based on your limited description, simply pool the data and calculate the mean and SD for the whole.  
