The TIMSS 2015 science achievement results are reported as average scores and distributions on the fourth and eighth grade TIMSS 2015 science achievement scales. Each scale summarizes students’ performance on a large number of test items (176 at fourth grade, 220 at eighth grade) designed to measure the breadth of science content, as well as a range of cognitive processes within the knowing, applying, and reasoning domains. The TIMSS Assessment Frameworks and items are developed and reviewed through a consensus process involving all participating countries.
The TIMSS science achievement scales were established in TIMSS 1995 based on the achievement across all participating countries, treating each country equally. At each grade, the scale has a range of 0–1,000 (although student performance typically ranges between 300 and 700). A centerpoint of 500 was set to correspond to the mean of overall achievement in 1995, with 100 points set to correspond to the standard deviation. Achievement data from subsequent TIMSS assessments have been reported on these scales so that increases or decreases in average achievement may be monitored across assessments. TIMSS uses the scale centerpoint as a point of reference that remains constant from assessment to assessment.