Class size is of particular interest to education researchers and administr
ators because it is one of the few variables that administrators can change
from term to term. In studies of class size, however, little if any attent
ion is given to the consequence of missing student records that result from
"data cleaning" done by those collecting the data, student unwillingness t
o provide data, or students self-selecting out of the study and the implica
tions of this selection on an appropriate measure of class size. These shor
tcomings are addressed here: class size and other class-specific variables
that may affect student learning of economics are considered along with the
hazard of attrition between the pre-course test and the post-course test a
nd students' failure to complete questionnaires about themselves and the co
urses. Contrary to studies that have used an average or an end-of-term clas
s size measure and find no class-size effect, beginning class size is found
to be significant and negatively related to learning of economics, all els
e equal. In part, this is the result of students in larger classes being si
gnificantly more likely than students in smaller classes to withdraw from t
he course before taking the posttest. (C) 2001 Elsevier Science Ltd. All ri
ghts reserved.