Not long ago, the ACT surpassed the SAT as the most widely used college admission test in the country. But the president of the testing organization that leads the market advises students to take whichever exam suits them — or both.

ACT President Jon L. Erickson, in Washington this week, spoke with The Washington Post about perceptions of the two tests among students and higher-education professionals.

About 1.8 million U.S. students in the Class of 2013 took the ACT, compared with 1.5 million who took the SAT. (The SAT also has a significant number of overseas customers.)

The ACT, launched in 1959, is 33 years younger than the SAT. Its calling card from the beginning was measurement of student achievement and alignment with school curriculum. The SAT, by contrast, started as an aptitude test that was explicitly not grounded in the curriculum. Now the SAT has dropped any references to aptitude and is marketed as an achievement test, too.

For generations, the ACT was clearly the No. 2 test in the admissions world.

That changed in the past decade as numerous states started requiring 11th-graders in public schools to take the ACT. In the next school year, 17 states will have such an arrangement. The ACT became the market leader in 2012. The SAT still leads in Maryland, Virginia and the District of Columbia, but the ACT is gaining users rapidly in the Washington region.

Use of the SAT fell in 29 states after the College Board changed the test’s format in 2005. Now the College Board, based in New York, has embarked on a major redesign of the SAT to take effect in early 2016. The ACT, based in Iowa, announced Friday that it will make its optional essay question more challenging starting in 2015.

The core format of the ACT is about three hours of multiple-choice questions in four sections — English, reading, mathematics and science — producing a total score of 1 to 36. The optional essay, which takes a half-hour, is scored separately on a scale of 2 to 12. Some students who take the ACT in 2015 will also see additional “constructed-response” sections, requiring written answers.

Here are excerpts from an interview with Erickson.

Post: Why should someone choose to take the ACT?

Erickson: I would say choose a test that you’re most comfortable with. And some students are more comfortable with one or the other. Depends on the format, depends on the content, depends on the opportunity to take it. Depends on the schools, if they have a true preference. But more so, I’d say consider taking both because you may do better on one than the other.

Post: Really? That’s a diplomatic answer. But why should one take the ACT?

Erickson: I think the ACT is a fair exam. I think the ACT is more comfortable. It’s what a student experiences in school, and so there’s not an anxiety, or an unknown gamesmanship to it. I think the ACT allows a student to look at themselves across a broader range of skills, including science.

Post: There’s a notion out there that the ACT is “easier” than the SAT. Is this true?

Erickson: There’s a notion from some people probably that the ACT is harder. It’s all in the eyes of the beholder. The ACT is a challenging exam, and it has a full range of questions. But there’s no evidence to suggest one’s easier than the other. It may be, if you don’t like science, it may seem like a harder exam. If you like science, or you’re good at math, it may be a more friendly exam. So it kind of depends on the student’s perspective.

Post: Some people liken this to Hertz and Avis, or Coke and Pepsi. ACT and SAT. Is that a fair comparison, that essentially this is a market where you have two equal, jump-ball choices?

Erickson: I don’t ever like to look at it like that. First of all, I think it’s a mistake to look at everything as a competition. You start comparing yourself to one other and things change around you, and you’re both out in the cold after that. We try to look very hard at what’s good for students, what do students want, what do schools want? How do we develop something that serves all of these audiences in a friendly way and also provides valuable information and data that’s reliable and valid. As long as we keep improving, which is our mantra, the rest of it takes care of itself.

Post: There is a school of thought that says: “Yes, you can take either one. But if you really want the best shot of getting into the most elite universities, you ought to think about taking the SAT.” How much is this true?

Erickson: It’s totally false. And I guess I don’t know what that school of thought means and where that comes from, but that’s not true.

Post: Not true at all?

Erickson: Not true at all.

Post: It wasn’t always the case that either test was equally accepted, right? You have worked your way into this market in some ways.

Erickson: It’s generational. There are counselors, writers, admission officers, teachers who grew up with that line of [thinking]. Sometimes it takes awhile for that to flip. Our biggest growth is actually from students on the East Coast now, but it’s taken a long time for people to acknowledge that. The on-the-ground behavior was there, but from the pundits, they didn’t see it. ACT’s a humble Midwestern organization in its roots. We’re just not self-promoting. That’s okay with us.

Post: So from everything you see and believe and hear, there is absolutely no disadvantage for a kid who wants to get into the most elite schools in America in taking the ACT instead of the SAT.

Erickson: Every admissions officer I talk to, and every policy I see, there is no disadvantage. There isn’t an institution in the country that requires the SAT instead of the ACT.

Post: Why did this change, and when did it change?

Erickson: It’s been changing over time. In the last 10 years, it really picked up speed. About five years ago, I think there was the last school that even had a preference.

Post: Which one was that?

Erickson: I think Harvey Mudd [College] was the last one to actually make a switch. I think it was a revelation — one, if you’re truly trying to reach out to students from everywhere, and have equal access or greater access for students, we need to consider both exams. There are two tickets for those students right now. The second is, there was an acknowledgment that the number of students taking the ACT was gigantic. And it’s a missed market for colleges if they aren’t thinking along that line. So frankly I think it was as much economically driven as it was opportunity-driven.

Post: So have you kind of snuck up on us here on the East Coast?

Erickson: I guess you can answer that better than I can. I don’t think we’ve snuck up. We haven’t been invisible.

Post: There’s another critique of the test that comes from the critique of testing itself. This critique says, “A pox on both your houses. Who needs these tests?” A growing number of colleges say testing is optional. What is your response?

Erickson: There’s some test fatigue going on. I think there’s also been an over-focus, an over-reliance, perhaps, on a test as a sole measure, as a sole determinant, as a sole indicator. And we, to our bone, reject that notion. We think testing is one component of a good decision. We can gather a lot of comparable and important information from testing. But it [has] to be in the context of students’ goals, interests, behaviors, other measures, grade points, teacher evaluations. Making any decision in life, you should have a lot of information to draw from — because some of it may be contradictory. Making a decision on one piece, like a test, is a dangerous practice.

Post: Wesleyan University, Wake Forest University — here in Washington, American University. Bates College. These are not small names. [They are test optional.] Are they wrong? Do they know something you don’t?

Erickson: I think they know what works for them at their institution. I will note that institutions that go test-optional still utilize tests, and students still have the option of sending them. In many cases, [these colleges] still receive lots and lots of tests. And they use that information for recruitment in a whole variety of ways.