Project managers need timely feedback on the quality of development products to monitor and control project progress. Inspection is an effective method to identify defects and to measure product quality. Objective and subjective models can be used to estimate the total number of defects in a product based on defect data from inspection. This paper reports on a controlled experiment to evaluate the accuracy of individual subjective estimates of developers, who had just before inspected the document, on the number of defects in a software requirements specification. In the experiment most inspectors underestimated the total number of defects in the document. The number of defects reported and the number of (major) reference defects found were identified as factors that separated groups of inspectors who over- or underestimated on average.