The average score of the countries whose 8th graders participated in TIMSS math in 1995 was 513, compared to an average 1999 score of 508 for those countries whose 8th graders took TIMSS math in both 1995 and 1999. This 5 point decrease can be explained by the fact that the higher scoring countries like Switzerland, Austria, France, and Ireland, and Belgium didn't participate in the test in 1999. But the average score of all of the countries whose 8th graders took TIMSS math in 1999 was 487, which is 26 points lower than the average for 1995. Even with the addition of though higher scoring countries like Taiwan, Finland, and Malaysia in 1999, the addition of countries like Chile, Indonesia, Jordan, Macedonia, Moldova, Morocco, the Philippines, Tunisia, and Turkey, all of whom scored well below 500, reduced the overall average TIMSS score.

Also, where Singapore had scored 143 points higher than the US in 1995, they scored only 102 points higher in 1999, and where Romania had scored 18 points lower in 1995, they scored 30 points lower in 1999. Relative to the US, the higher scoring countries scored an average of 22 points lower in 1999 than they did in 1995, but the lower scoring countries scored an average of 2 points higher.