Distribution of Average Combined Reading Literacy Scores

The average scores for reading literacy describe how a country performs overall compared to other nations, but they provide no information about the way scores are distributed within the countries. One country with an average score similar to another could have large numbers of high- and low-scoring students, while the other country could have large numbers of students performing at about the average score. Figure 4 details how scores were distributed across countries.

• In the United States, the 5th percentile score for combined reading literacy is 389. Ninety-five percent of U.S. students score above 389; in the same way, 5 percent of students score above 663, the 95th percentile score. This means that the top 5 percent of U.S. students score at least 274 points higher than the bottom 5 percent (figure 4).

Looking at the length of the bars in figure 4 gives a sense of how large the differences between a country's highest and lowest performing countries, but it does not describe how many students are high or low performing. As with average scores, because of the statistical techniques used to sample students, it is not accurate to rank countries' scoring variation based simply on the length of the bars shown in figure 4. Standard deviations of the combined reading literacy average scores gives a mathematical way to tell how greatly scores are spread out from the country's average score.

• Seventeen countries, or about half of the countries participating in PIRLS 2001, show less variation in student performance than the United States. Ten countries show a higher variation, while the remaining eight countries show no detectable differences in variation in student performance compared to the United States.