But one does not need to look deeper into the exam results. One simply has to assume a wider perspective. First, it is not just the average that counts, the distribution is very important. For example, below is the distribution of scores for the 4th grade science test.
|Above copied from TIMSS 2015|
The TIMSS report also comes with an analysis of scores that provides correlations with various factors. The following are factors observed to correlate strongly with 4th grade science scores. Similar conclusions are reached with the math scores.
First, socio-economic status correlates with scores: