Esperanto in Scotland, Chapter 5, Exercise 1
Sample answer (Jarlibro, the yearbook).

A yearbook lists learners who chose to be listed, in the year it was compiled. Limits: it
misses learners who did not send in their details (the study says the Jarlibro did not
capture all learners); it shows addresses, not how active people were; and listed numbers
can be out of date or inflated by enthusiasm. It is good for mapping where groups existed,
poor for counting people exactly.

(Other answers: Zamenhof's directory covers only 1889-1908 and only those who sent a coupon;
magazine notices cover only people who advertised; introductions from pen friends follow
existing networks and reproduce their gaps.)