Program comprehension is an important human factor in software engineering. To measure and evaluate program comprehension, researchers typically conduct experiments. However, designing experiments requires considerable effort, because confounding parameters need to be controlled for. Our aim is to support researchers in identifying relevant confounding parameters and select appropriate techniques to control their influence. To this end, we conducted a literature survey of 13 journals and conferences over a time span of 10 years. As result, we created a catalog of 39 confounding parameters, including an overview of measurement and control techniques. With the catalog, we give experimenters a tool to design reliable and valid experiments.