Developed by Al Wonderlic, a Northwestern University psychology graduate student in the 1930s, as a way of measuring employee potential, the test was first used by NFL teams in 1970.

Along with criminal background checks and rigorous physicals, it is now a standard feature of the process through which they try to decide which players to select in the annual draft.

An estimated 2.5 million other Americans sit down for the exam every year at various corporations but it's only when 330 of the brightest grid-iron prospects are handed their pencils that the rest of the world takes notice.

This may be because, over the course of nearly four decades, only one athlete, Harvard graduate Pat McNally, has ever notched a maximum score.

"It really did seem like an easy test at the time, " said McNally who, after spending 10 years punting in the NFL for the Bengals, now works for Wonderlic. "One of the reasons I did so well is because I didn't think it mattered. So I think I didn't feel any pressure at all.

It was more of a lark, and that's when you do your best. If I took it 100 times I'd probably never do that again. I didn't find out till years later that it hurt me in the draft. Coaches and front-office guys don't like extremes one way or the other, but particularly not on the high side. I think they think guys who are intelligent will challenge authority too much."

The credibility of the exam is regularly called into question. Twelve months have passed since the then university of Texas quarterback Vince Young was rumoured to have scored a rather embarrassing six the first time he took it. As the key player on every team, quarterbacks usually score an above-average 24. Worse again was the fact Young allegedly only managed to upgrade to 16 at his second time of asking. When those results were leaked to the media, he endured the ridicule of a sporting nation. Eventually drafted by the Tennessee Titans, he proved devotees of the test wrong by having a magnificent debut season that culminated in him winning NFL offensive rookie of the year.

"So what was that test worth?"

said Doug Williams, a retired quarterback who is currently working for the Tampa Bay Buccaneers.

"Some people use it, but I've always said, some players have ball sense who can't take tests. That's been proven over the years. Not everybody is a good test-taker. That's why you have the regular universities out there and you have the Harvards."

History bears out Williams's argument. Dan Marino (right) may well be the greatest quarterback ever to play the game and famously, he scored a paltry 15 on the test. According to Wonderlic guidelines dictating that a 10 represents somebody with basic literacy and a security guard should be able to manage an 18, this meant Marino had an IQ suited to a job requiring very little thinking on his own. Hardly an appropriate description for a man whose swift decision-making enabled him to throw for more passing yards than anybody else in history.

At the other end of the scale, a high score in the classroom is no guarantee of future success either. A highly-touted college quarterback named Drew Henson managed a 42 in 2003 and he was last heard of still honing his game in the backwaters of NFL Europe.

Similarly, New York Giants' quarterback Eli Manning's 39 was a whopping 11 points smarter than his Super Bowlwinning brother Peyton yet the younger sibling remains an unproven quantity at the highest level.

Examples like those are exactly why critics maintain that, as a tool for evaluating personnel, the Wonderlic works better in ordinary jobs than it does in the crucible of professional sport. Still, clubs remain in love with the exam. It seems that in their quest to try to take the guesswork out of predicting future stars - not to mention attempting to justify splurging millions of dollars on unproven talent year after year - some gauge of intelligence is better than none at all.

"What the score does is help match training methods with a player's ability, " said Charlie Wonderlic, president of Wonderlic Inc, in an interview with ESPN. com. "It could be a playbook - what is the best way to teach a player a play? On the field, the higher the IQ, the greater the ability to understand and handle contingencies and make sound decisions on the fly."

A boy is 17 years old and his sister is twice as old. When he is 23, how old will his sister be? Answers on a postcard to the NFL club of your choice.