There are a lot of different lines of mice used in the lab. Generally the incidence rate of cancer is pretty consistent in the lines used for cancer research.

There are a lot of different lines of mice used in the lab. Generally the incidence rate of cancer is pretty consistent in the lines used for cancer research. In addition, most cancer studies test for whether there is an increase in cancer rates from the baseline for the mice. For example, you'd purchase 100 mice all from a specific breeding line. You'd randomly choose 50 of those mice to expose to experimental conditions. The remaining 50 would be treated in exactly the same way except for the variable being tested. This is the control group. For example, sticking with aspartame. You'd take 100 mice and randomly select 50 to be exposed to aspartame. All 100 mice would be kept in the same type of cage, receive the same amount of the same food and water from the same source, but the 50 experimental group mice would have aspartame added to the water. Even better would be if there was a way to have them consume the aspartame in controlled amounts and monitor food and water intake to track whether they eat and drink more or less in response to the aspartame, as that could also potentially effect the cancer rate. It's not as simple as mice and rats are prone to cancer. They are. The people running these experiments are well-aware of that and go to great pains to ensure that that is not what's being measured.