[R] Difference scores

[statscurious]

Hi all,
I have a couple of questions that are general statistics questions rather
than being R-specific.

I'm interested in figuring out how to compute something like standard error
for difference scores (in particular, differences scores of reaction times). 
Does anyone know if there is a standard way to do this? Any references would
be very much appreciate.

Secondly, I'm interested in figure out something like standard error for
data points contributing to a correlation.  I am looking at the correlation
between the means of reaction times in two tasks by individual participants,
and for some participants, the scores are going to have more or less error
associated with them (and it is also possible that task A might have more
error associated with it than task B, etc.). It seems logical to weight each
set of points by this error when computing the correlation somehow, but I'm
not sure if there might be a standardized way of doing this. Does anyone
have any ideas or know of a method that is typically used?

Any pointers are greatly appreciate, thanks very much!

M
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