Crosby’’s recent study of American historical demography is blithely based on the reconstitution of the records of single parishes, a method that often excludes migrants. Moreover, it is troublesome for historians to obtain information on the birthdates of people who relocated to the parish, and equally difficult to follow those who had migrated to new places of residence. Thus, the exclusion of migrants also followed from the way spatial units were once conceived by the parishioners themselves, a stable and unchanging pre-modern countryside of interchangeable towns unlike "modern" flows to cities. 　　As a result, migration was improperly assumed to be irrelevant because the small units in the countryside were interchangeable and migrants into a parish could thus stand as a proxy for those who had left. In any case, it was thought that migration in the countryside was repetitive and occurred only in response to life course events, such as finding a spouse, and thus, like the parishioners themselves, Crosby complacently equates the demographics of migrants to those of more sedimentary populations.

The passage suggests that one major difficulty in establishing patterns of migration is________.（）