The SAT remains the most commonly used college entrance exam -- with about three million students taking the test each year, according to the College Board. The popularity of the ACT, however, is increasing, and 1.5 million students took the test in the 2009-10 school year -- up from just less than a million in the 1996-97 school year, with numbers steadily rising every year in between, according to the National Center for Education Statistics. The ACT is designed to produce standardized results across populations, but while it can measure college readiness in some students, it does not measure all factors.

Aptitude vs. Achievement

Unlike other standardized tests, such as IQ tests and the SAT, the ACT is designed to be an achievement test. Rather than measuring generalized aptitude, it measures the specific knowledge a student has mastered in high school. A student's ability to learn in high school is a significant predictor of her ability to learn in college. Moreover, achievement tests can measure subtle factors that contribute to learning. For example, a very intelligent student who never does her homework might perform well on the SAT, but do poorly in college because she is unable to work. Because the ACT measures specific knowledge, however, the same student might do poorly on the ACT.

College Readiness Measures

The makers of the ACT provide statistics on college readiness broken down by category. In 2011, only 25 percent of students who took the ACT were ranked as ready for college in all four domains of knowledge that the ACT measures. In the 2009-10 school year, students scored lowest in science readiness, with only 29.3 percent of students being ready for college science classes. Students scored highest in English, with 66.3 percent ranked as ready. These relatively low readiness rankings produced by the ACT might reflect the general lack of readiness many high school students face when applying to college and could accurately predict potential problems with college performance.

Test-Taking Skills

While the ACT purports to measure achievement, it also measures test-taking skills, which are a major component of college success. Students who can test well often get higher grades, particularly on multiple choice tests. Conversely, test-taking is only one portion of college achievement. Regular class attendance and participation, critical thinking skills, time management and other factors can also affect college performance. The ACT can't measure these skills and therefore a student's ACT score can end up either underestimating or overestimating her potential college performance.

Test Flaws

The ACT can't fully measure a student's aptitude or achievement. Cultural bias is a problem with most standardized tests, and the ACT is no exception. Students from different cultural backgrounds might understand test questions differently and be unfamiliar with colloquial phrases or even household objects used in word problems. Stereotype threat can also play a role in test performance. Minority students, such as women and students of color, tend to score lower when they are reminded of negative stereotypes about their group. Simply checking a box indicating race and gender can serve as a reminder of these stereotypes, and the ACT requests this demographic information. Consequently, some students might underperform on the test and do much better in college than their test results might predict.