score:27
The Oxford Dictionary of Quotations has the following citation for that quote:
(apologies for the limited quality of the scan. I'm using my hand-held scanner.)
As their source, they cite The Life, Letters and Labours of Francis Galton by By Karl Pearson (p415):
Interestingly, unless he is quoting from a source that he does not cite (such as a letter from Florence Nightingale to Francis Galton), it appears that Karl Pearson is putting those words into Florence Nightingale's mouth.
If that is, in fact, the case then it appears the source of the quote is Karl Pearson rather than Florence Nightingale!