In general, the sampling plan for the TIMSS 1999 Video Study followed the standards and procedures agreed to and implemented for the TIMSS 1999 assessments. The school sample was required to be a Probability Proportionate to Size (PPS) sample. A PPS sample assigns probabilities of selection to each school proportional to the number of eligible students in the eighth -grade in schools countrywide. Then, one mathematics and/or one science eighth-grade class per school was sampled, depending on the subject(s) to be studied in each country. Only one class was randomly selected within each school. No substitutions of teachers or class periods were allowed. The designated class was videotaped once, in its entirety, without regard to the particular mathematics topic being taught or type of activity taking place. Teachers were asked to not prepare special lessons for the videotaping; rather, they were asked to conduct the lessons they would normally have given on the day of videotaping, and to deliver it in the way they would normally as if they were not being videotaped. The only exception was that teachers were not videotaped on days they planned to give a test for the entire class period. The final sample of classes captured on videotape was reviewed by an international sampling referee. Weights were developed for each country's data to account for sampling error and to allow for appropriate national estimates. Finally, to determine whether the final sample of teachers included in the study were generally representative of teachers of eighth-grade classes in their respective countries, the teachers were asked a series of questions, such as their academic preparation, years of teaching experience, certification status and teaching load, that could be cross-referenced with other known data sets to determine the representativeness of the final sample. Analysis of the data indicate that, in general, the teachers selected for inclusion in the study in each country are similar to the larger population of teachers who teach at that grade level.