Within the broader literature concerned with potential bias in student measures of instructor effectiveness, two broad types of bias have been shown to operate in a course: internal and external. Missing is an assessment of the relative influence of each bias type in the classroom. Do internal or external types of bias matter more or less to student measures of instructor effectiveness? This question is of interest to those who recognize that many forms of bias operate in the classroom simultaneously and to great effect on course evaluations. Through the combination of quasi-experimental and survey methods, we attempt to do something not done before--simultaneously measure and compare the effects of one internal and one external type of bias to which we refer, following convention in the literature, as an internal factor and an external factor. Our findings indicate that, while both factors studied are important, perception of instructor partisanship (internal factor) is more influential on student course evaluations than our external factor (experimental treatment--literally a chocolate treat). The upshot is that American government instructors should be especially careful stating partisan positions in class.