I’ve just finished reading Freakonomics and my overall impression is that it could have done with an appendix on statistical analysis, regression analysis in particular. The later chapters appeal to regression analysis a number of times and the one serious lack is an explanation of what that means.
Readers familiar with statistical analysis will not find this a problem, but this is not a book aimed at statisticians and economists. It is (as I see it) a book aimed at the reader of popular science, the reading public. Such an appendix might not have been bigger than the section of notes (some 21 pages), but would be provide a lot more support, in the mind of the casual reader, for the author’s arguments and interpretations than the lists of citations, references and sources.