A basic query. We are doing a meta analysis where we are gathering estimated relationships from a series of papers. Each of these studies estimates two parameters of interest say b1 and b2 and reports their standard errors as usual. We are interested in the quantity (b1 - b2). My understanding is that we cannot derive a standard error for (b1 - b2) simply from the standard errors for b1 and b2 - we would also need cov(b1, b2) (which are not reported of course). Correct? And just taking the square root of the sum of squared standard errors would not be an acceptable approximation, yes?
Thanks for any insight, and apologies for the basic nature of this query!