Which aspect of the flea control experiment helps ensure that results are valid?

Random assignment to treatment groups is crucial in experiments because it helps to eliminate biases and ensures that any differences observed between the treatment and control groups can be attributed to the treatment itself, rather than to pre-existing differences among the subjects. By randomly assigning dogs to either the flea treatment or a control group, researchers can ensure that factors such as breed, age, and health status are evenly distributed across both groups. This process increases the internal validity of the study, allowing for more reliable conclusions about the effectiveness of the flea control method being tested.

Other factors, such as the choice of using only dogs or the setting of the study, do not address potential biases in how subjects are assigned to treatments. While conducting the study in a laboratory can provide controlled conditions necessary for certain aspects of research, it does not inherently validate the experimental results in the same way that random assignment does. Collecting data over several months could help capture long-term effects, but without random assignment, the results could still be confounded by variables not controlled for, impacting their validity.