From one point of view, the U.S. Census Bureau offers a glowing report on America's goal of becoming an educated nation.
A record 81 percent of the population has completed high school or its equivalent, and the percentage appears to be headed still higher.
But another number tells of trouble on the horizon. Among the most recent crop of young adults, those now 18 to 24, only 75 percent have finished high school, suggesting a substantial dropout rate.
Therein lies a mystery: Why is the national graduation rate climbing while so many school-age children are dropping out?
The question evokes an array of answers from demographers, statisticians and educators. Some think immigration of less-educated young people is skewing the numbers. Others point to "redshirt" students who spend an extra year in high school, then graduate at age 18 or 19. They also cite "second-chance" dropouts who earn degrees in their later years by passing an exam.
That the Census Bureau seems to be pointing in both positive and negative directions is not just a matter of academic curiosity.
Critics contend that the government is inflating the nation's educational achievement by counting people who pass equivalency exams as essentially the same as those who graduate from high school.
But the one factor that probably contributes most to the conundrum stirs hardly any mention: The graduation rate is almost guaranteed to climb as long as people born before 1940 continue dying.
The oldest generations have the lowest rate of high school completion in the nation. In 1940, only one-fourth of the adult population had at least 12 years of schooling. Since then, the rate has climbed steadily by an average of almost 10 percentage points a decade, according to the Census Bureau's Current Population Survey, which takes an annual look at educational attainment.
"The completion rate of your total population [age] 25 and over will continue to increase as long as those coming in have higher levels than those going out," said Jennifer Day, chief of the Census Bureau's education and social stratification branch.
Day estimated that the trend could hold for another census or two.
New immigrants with little or no schooling are also part of the picture. Some of them will earn high school diplomas through adult school or obtain equivalency degrees.
For those reasons, the Census Bureau monitors a later age group, 25 to 29, as the bellwether for American educational attainment.
"We have close to 90 percent of students graduating from high school or having an equivalent by the time they are 25," Day said.