A particular strength of such studies is the fact that potential confounding variables such as socio-economic status, occupation, and smoking do not vary on a day-to-day basis in an individual area.
The same amount of people smoke in Boston each day.
mnli hypothesis: The same amount of people smoke in Boston each day. premise: A particular strength of such studies is the fact that potential confounding variables such as socio-economic status, occupation, and smoking do not vary on a day-to-day basis in an individual area.