U.S. students are generally improving in math and science, along with their peers around the globe, but the latest Trends in International Mathematics and Science Study results—including a longitudinal look for the 20th anniversary of the tests—show more of a slow uphill slog than a breakout performance.

"The [United States] is a large and diverse country, so it's difficult to see a large increase over a short time," said Michael O. Martin, a co-executive director of the International Association for the Evaluation of Educational Achievement's TIMSS and PIRLS International Study Center at Boston College. The center has conducted the TIMSS in the United States, with the National Center on Education Statistics, every four years since 1995.

The TIMSS is based on specific content areas, and NCES Acting Commissioner Peggy Carr noted that U.S. students' performance in different math topics varied significantly on the test over time. While 8th graders' performance in geometry and algebra improved significantly from 2007 to 2015, their scores flat lined on number theory and actually declined significantly on problems of data and chance. Similarly, U.S. students improved significantly in their performance on life science and biology topics, but their scores in physics and earth sciences stagnated.

By and large, though, countries participating in TIMSS have increased the depth and rigor of their math and science curricula over time, according to Ina V.S. Mullis, co-executive director of IEA's TIMSS and PIRLS International Study Center. "When we started [conducting TIMSS] in 1995, our math was all content—algebra, geometry—and in science, chemistry, physics," Mullis said, "but now we also include cognitive demands, thinking skills ... school is getting to have a broader dimension that is quite different than it was 20 years ago."

Among the new findings:

Mathematics

U.S. 4th graders scored on average better in math in 2015 than they did a decade or two ago, but their performance has been flat since 2011. That puts the United States above average internationally, with students in 10 countries performing better and those in 34 countries performing worse in average math scores in 2015. However, the gains since 2007 have come from the above-average students. Those who performed in the bottom 25 percent in 2007 have actually had lower scores since then.

U.S. 8th graders scored higher in math than they have in the test's 20-year history, again putting these students in the top half of countries tested. And again, these gains were driven by gains among students who were already performing above average:

To put those scores into context, in 8th grade, 20 scale-score points was roughly a school year's worth of learning, according to Dirk Hastedt, the executive director of the International Association for the Evaluation of Educational Achievement.

In the TIMSS Advanced, which assesses math concepts such as calculus and geometry among 18-year-olds, U.S. average math scores were higher than those of any other country except for Lebanon and students in an intensive program in the Russian Federation (The general Russian Federation sample equaled U.S. performance levels.) Overall, though, both the U.S.'s and other countries' students performed worse on advanced concepts in 2015 than in 1995:

Science

U.S. students significantly closed gaps between male and female students at both 4th and 8th grades in science.

"I was really surprised to see the narrowing of the gender differences" in the last 20 years, Martin said. "The differences in math have been greatly reduced; we are on our way to completely eliminating the gender gap in science."

However, average scores in the United States improved in the last decade but not since the last testing cycle in 2011, at either 4th grade:

or 8th grade:

While U.S. students did not improve on the TIMSS Advanced in science, 75 percent more students were at least able to take the test than did so in 1995.

"That basically means a significant increase in the number of students who have physics being offered to them, and ... the scores in that area remain pretty high. We should be encouraged," said David Evans of the National Science Teachers Association.

However, Evans argued that the United States has not shown a strong public interest in science or science education compared to higher-performing countries such as Singapore, Korea, Japan, or Sweden. "I have to attribute it to the social climate. In those Asian countries ... there's a much greater social pressure and desirability to support science and math than there is here."

Who Did TIMSS test?

TIMSS includes a representative sample of more than 310,000 4th grade students in 55 countries and education systems (like Chinese Taipei), and more than 260,000 8th grade students in 44 countries and education systems. In the United States, that amounted to about 10,000 students each in 4th and 8th grades, in roughly 250 schools at each grade. Florida provided a separate sample of more than 2,000 students at 4th and 8th grades in more than 50 public schools statewide, though its students' test performance generally mirrored that of the United States as a whole.

The separate TIMSS Advanced tested about 56,000 18-year-olds in France, Italy, Lebanon, Norway, Portugal, the Russian Federation, Slovenia, Sweden, and the United States. TIMSS Advanced has previously been administered in 1995 and 2008, but the United States did not participate in 2008.

I'll be digging into the TIMSS data on top-performing countries and equity issues over the years later today and this week.

Charts Source: National Center for Education Statistics