Another important step forward in understanding cancer happened in 1926, when Janet Lane-Claypon published a paper on cancer epidemiology. (Epidemiology is a field of study which looks at how common a disease is, what patterns the disease takes in different kinds of people, and what this means for understanding and treating the disease.) This historic paper was a comparative study, which tries to find out what causes a disease by looking at a group of people who have the disease and figuring out how they are different from another group that does not have the disease. Lane-Clayton's study looked at 1000 people who all had the same background and lifestyle (or way of living): 500 people with breast cancer and 500 control patients (people without breast cancer). These people were the same in many ways, but some got breast cancer and some did not. To figure out what might be causing certain people to get breast cancer, the study looked at what was different about these people when they were compared to (or looked at alongside) the people who did not get cancer.