Searching the net for << "noise reduction" averaging factor >> gets us lots links to "solid work". For example:
From
http://www.owlnet.rice.edu/~dodds/Fi...oise_notes.pdf
The generally accepted improvement in SNR is the square root of the number of averaged images. That is to say that the mean of e.g. 9 images will have an SNR 3x "better" than any one of those nine. The quote above mentions "complications". One complication would be the calculation of the SNR for the long exposure versus the SNR for the shorter exposure.
Anybody?
The wording of Manfred's post implies serious doubt as to any benefits of averaging multiple exposures.
This whole discussion suffers from the use of the single term "noise" without qualification and the lack of actual numbers or examples.
Pardon my pedantry ...