Yule ran a regression of changes in pauperism on changes in the out-relief ratio, with changes in population and changes in the population aged 65+

as control variables. He used data from three censuses and four strata of unions, the small geographical areas that administered poor-law relief.

He made a causal inference: out-relief increases pauperism. To make this inference, he had to assume that some things remained constant amidst changes. Can you explain the constancy assumptions?