This analyses the effects of unplanned item nonresponse on the measurement of student ability in large scale educational achievement studies. It explores variations in student and national ability estimate distributions as a result of different psychometric treatments of nonresponse. A range of replicated analyses are presented using data collected from the crossnational SACMEQ study of Reading achievement. The relative merits of these treatments are summarised in conclusion, and implications for normative and methodological research and practice are considered. It is suggested that environmental and psychological antecedents of nonresponse need to be determined and that related variables be considered during the production of individual and national estimates of achievement.