M. S. Bartlett Quote

[Statistics] is concerned with things we can count. In so far as things, persons, are unique or ill-defined, statistics are meaningless and statisticians silenced; in so far as things are similar and definite — so many workers over 25, so many nuts and bolts made during December — they can be counted and new statistical facts are born.

Essays on Probability and Statistics, Some Remarks on the Theory of Statistics (p. 11)