It is argued that to evaluate an information source (e.g., a Wikipedia article), it is necessary to relate the content of that source to an interpretation of the state of knowledge at the research front (which is typically developing dynamically). In the research literature, there is a controversy about the effect of screening programs for breast cancer. This controversy is used to compare the value of Wikipedia with Encyclopedia Britannica and two Danish encyclopedias as information sources. It is argued that this method of examining information sources is preferable to other methods which have been suggested in the literature.