[Rivet] normalisation to cross-sections

[Hendrik Hoeth]

Thus spake Frank Siegert (frank.siegert at durham.ac.uk):

> Sorry, one minor correction that I realised just now sitting in the 
> seminar...
> 
> Frank Siegert, Friday 23 October 2009:
> > If I normalise
> > histograms to 1 and I have filled _weighted_ events into these
> > histograms, this doesn't work, because there might be runaway bins
> > which create a bias towards low values for all other bins.
> 
> This isn't limited to _weighted_ events after all.
> The main point when this appears is when a histogram is filled after
> very strict cuts, such that the integral of the histogram might be
> quite unstable from run to run.

Okay, we've just had our mails crossing.  ;-)

I'd say: In unweighted events there is a way to get around it
(reconstructing the sumofweights from the errors), while in weighted
events that's not an option.

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