Use a No. 2 pencil, fill out each circle completely, and don't make any stray marks—these are rules every child learns when taking standardized tests that are largely based on multiple choice. But are multiple choice exams the best way to test students' knowledge?

Experts say they are—sort of.

The College Board, which designs and administers the SAT college entrance exam and the Advanced Placement subject tests, has tried to make multiple choice questions play less of a dominant role in its exams. In 2005, the organization added a free-response writing section to the SAT, and the number of multiple choice questions on its AP science exams will be reduced from 100 to 55 over the next several years, as the free-response section is expanded from four to seven questions.

[Learn more about the new AP science curricula.]

But multiple choice and other objective test questions, which have one definitive answer, are likely here to stay. They're quick, for one thing, testing students on a wide range of material in a short period of time. They're also cheap, says Robert Schaeffer, public education director of FairTest, the National Center for Fair & Open Testing, an organization that "works to end the misuses and flaws of standardized testing."

Cost is important in an environment where federal and state governments are continually mandating more testing. "Experts know [multiple choice questions] are not the best way to assess higher-order thinking skills," Schaeffer says. But multiple choice tests are "easy to design, trial, score, and revise. You often see previously used questions with components changed."

When it comes to testing a wide range of topics, however, multiple choice questions are useful.

"They allow us to cover a lot of content," says Wayne Camara, the College Board's vice president of research. Essay and free-response questions measure a student's knowledge in more depth, which is a good thing, Camara says. But if a student glosses over a topic while studying and that topic shows up on an essay question, they can be out of luck, even if they know a large percentage of the course material.

"If a student gets a four [out of five] today, we don't want them to get a two if they take a different form of the test in two weeks. That kind of test is not reliable," Camara says. And reliability, especially when college entrance or credit is at stake, is key.

[Read about falling SAT scores.]

By asking lots of multiple choice questions, test makers are able to more accurately judge a student's breadth of knowledge. Camara says it's important to get a mix of multiple choice and free-response questions to measure both surface-level understanding of the material and higher-level thinking skills.

"When you limit a student to one or two essays, you're not necessarily offering them an opportunity to do their best work," he says. "The [AP] test is a representation of an academic year. The more points of information you have, the more likely you'll get a reliable score."

However, Schaeffer, of FairTest, says multiple choice questions are useful for judging students' "factual regurgitation," but they're not particularly good at judging students' knowledge. If students can eliminate an incorrect answer or two, they can get a question correct without knowing the material.

The SAT math and verbal sections, he says, are "largely multiple choice games with a premium on strategic guessing." That's not good news for deliberate, methodical students. He says multiple choice exams discriminate against students whose "culture or style is not to be able to boldly guess and move on."

"You might argue that [being deliberate and methodical] are good traits to use in life," Schaeffer says, "but they're terrible traits on multiple choice tests."

[Get tips from the U.S. News College Test Prep guide.]

Tom Matts, College Board's senior director for assessments in the AP program, says that's why the AP science exams will feature fewer multiple choice questions. "Rather than covering the details and minutia of a broad range of content, there will be more attention paid to depth of understanding," he says.

The new AP biology exam, which will go into effect next school year, will also have objective questions that are not based on multiple choice. These "gridded" questions will be math-based and will require students to bubble in numerical answers after working out the problem.

Hybrid multiple choice and free-response tests like the AP exams are closer to ideal than purely multiple choice exams, Schaeffer says, and it's something the government should try to emulate. He says the National Assessments of Educational Progress (NAEP) are also relatively good models because only a sample of students take each test, and not every student has to take a full version of the exam.

"You get overall state accountability without subjecting every kid to a ton of testing," he says.