That’s an interesting issue.
The current ROOT “User’s Guide” (v5.26) mentions the “LL” option (but not the “WL” one) in the “Chapter 5. Fitting Histograms” in “The Fit Method” section:
http://root.cern.ch/download/doc/ROOTUsersGuideHTML/ch05.html#d5e4997
http://root.cern.ch/download/doc/5FittingHistograms.pdf
while “LL” is not present (but “WL” is there) in the current (ROOT 5.30 and newer):
http://root.cern.ch/root/html/TH1.html#TH1:Fit@1
though “LL” is still present (but not the “WL”) in ROOT 5.28:
http://root.cern.ch/root/html528/TH1.html#TH1:Fit
Just for the case something disappears completely one day: [quote]“LL” - An improved Log Likelihood fit in case of very low statistics and when bin contents are not integers. Do not use this option if bin contents are large (greater than 100).[/quote] [quote]“WL” - Use Loglikelihood method and bin contents are not integer, i.e. histogram is weighted (must have Sumw2() set)[/quote]
Some further explanations can be found in [url]Fitting histogram weighted with low stats “WL” option