A study compared performance on a silent informal reading inventory and achievement on a standardized reading test, focusing on students' individual skill needs and how effective tests were in identifying specific strengths and weaknesses in these skill areas. Subjects, 25 students entering fourth grade in a primarily middle class suburban community, were administered a silent informal reading inventory (IRI) after reading a passage. Subjects' most recent scores on the California Achievement Test (CAT) were compared to the results of the IRI. Results indicated that performance on a silent informal reading inventory do not correlate highly with achievement on a standardized reading test. Findings suggest that assessing student progress in reading ability for appropriate instruction is best achieved by a combination of the normed approach (as in the CAT) and a diagnostic-prescriptive approach (as in the IRI). (One table of data is included; 51 references, the evaluation instrument, data for reading comprehension instructional levels is appended. (RS)