To show the difficulty of eliminating test bias and to develop a methodology for distinguishing between the effects of prior knowledge and of skill development on reading comprehension, 207 eighth grade students from rural and urban areas were administered an 18-question reading comprehension test. Quantitative and qualitative effects of prior knowledge on reading comprehension were demonstrated through an examination of student performance on the test's different types of questions: (1) textually explicit--drawing on information directly stated in a single sentence of text, (2) textually implicit--requiring a synthesis of information, and (3) scriptally implicit--demanding background knowledge. The study suggests that test scores are biased by prior knowledge and reflect the students' I.Q. more than specific reading comprehension skills. The findings indicate that test bias can be lessened by asking central, rather than peripheral, questions on passages for comprehension and by using a content-specific vocabulary test to estimate the individual's prior knowledge. (MM)