The Different Economic and Social Conditions-Both Within Each Nation and Between

The different economic and social conditions-both within each nation and between them create a number of problems for researchers including:

A) the fact that data are gathered in different years.
B) the fact some countries do not pay people "income"
C) the condition whereby researchers do not include slavery in their calculations.
D) poor phone connections between the U. S. and other countries.
E) the universal unemployment in most African countries.