
TIMSS COVERAGE INDEX FOR FINALYEAR ASSESSMENT http://timss.bc.edu/timss1995i/TIMSSPDF/SRAppA.pdf A further difficulty in defining the desired population for the finalyear assessment is that many students drop out before the final year of any track. Thus a TIMSS Coverage Index (TCI) was calculated that quantifies the proportion of the entire schoolleaving age cohort that is covered by the TIMSS finalyear sample in each country. The TCI was defined as follows:
The numerator in this expression is the total enrollment in the grades tested by TIMSS, estimated from the weighted sample data. This estimate corresponds to the size of the population to which the TIMSS results generalize and makes appropriate provision for student nonresponse. It does not include students who are no longer attending school or students who were excluded from the sample on grounds of physical or other disability. It also does not include students who were repeating the final grade. Because some students repeat the final year of a track, or take the final year in more than one track at different times, they may be in the final year of a track without completing their secondary education that year. On the one hand, students who are not completing their education still have the potential to gain further knowledge in additional years of schooling, and thus will not have attained their full yield at the time of the TIMSS assessment. On the other hand, and of more serious concern, the presence both of students who are repeating the final track and of those who will repeat that track can contribute a substantial downward bias to the estimated achievement of the population. Repeating students would be represented twice in the population, and are likely to be lowerachieving on average than those who do not repeat. The only practical way for TIMSS to deal with this problem was to exclude students who were repeating the final year. Thus, the population of finalyear students is formally defined as those students taking the final year of one track of the secondary system for the first time. The denominator in the expression is an estimate of the schoolleaving age cohort size. Since the age at which students in uppersecondary school may leave school varies, TIMSS estimated the size of the schoolleaving age cohort by taking the average of the size of the 1995 age cohorts for 15, 16, 17, 18, and 19yearolds in each country. (Although the estimate was generally based on the 1519 age group, there were exceptions; for example, in Germany it was based on the 1719 age group.) This information was provided by NRCs from official population census figures in their countries. This approach reflects the fact that students in the final year of secondary school are likely to be almost entirely a subset of the population of 15 to 19yearolds in most countries. Table A.4 presents the computation of the TCI for each country. 

