IQ and psychometric tests have become popular tools in staff recruitment. And there are dozens of websites offering tests to let you see how you might measure up

It's been a roller-coaster week of self-discovery. A few nights ago, when preparing for a job interview, I stumbled across a website offering instant IQ tests.

Fearing that psychometric tests awaited me, I opted to take it. I should have known better. The hour was late, my brain was fuzzy and there was little more than caffeine propping up my eyelids.

I had barely begun when the automatic 10 minute-timer cut in. Seconds later, the results dropped into my mailbox. I was left staring at my screen in a mixture of horror and disbelief. I had scored 75, placing me on the lowly third percentile. The website's challenge to "make a killing brain skilling" was cruel. I needed counselling.

A sleepless night followed as I tried to account for my dismal performance, terrified that my Mensa-calibre partner should discover my secret. The job interview was the last thing on my mind: I had already thrown in the towel.

IQ and psychometric tests have become popular tools in staff recruitment. And there are dozens of websites offering tests to let you see how you might measure up.

Many use free tests to promote further paid-for examinations, or ask for money to give you a more detailed analysis of your results.

Fearing a degenerative brain disorder, I resolved to take another IQ test the next day. I chose the internet's "most popular" IQ quiz at IQTest.com.

I cleared my head, gulped back my coffee and, with heart thumping, clicked start. I raced through the questions, intent on recovering enough extra points just to lift me into the below average category.

I opened the results message and nearly fell off my seat. I had garnered an extra 55 points and been promoted to a borderline gifted genius. I was so relieved I almost parted with $9.95 to receive a full profile of my exalted intellect.

Suddenly, the job interview was a realistic proposition again. Further online IQ testing continued to throw up wild variations.

Why did this happen when many of the websites claimed their tests were accurate to within five points?

A cynical thought crossed my mind. Were some of the websites manipulating the results to sell their merchandise? Or were the variations simply down to inadequate test conditions, the use of different scales and the much-debated flaws of the IQ test itself?

I decided to try an experiment. I gave my partner the URL of three websites that had tested me and asked for his results or, rather, to spare his modesty, the variation in his scores.

He was happy enough with www.iqtest.com, which put him in its genius category. He had no quibble with the Guardian's test which placed him in its top 120+ category. But www.intelligencetest.com, which gave me such a deplorable score, found him below its admittedly high average of 119.

Mensa must be the ultimate authority on IQs but it wouldn't settle this puzzle. You can't qualify for its brainy ranks via an internet test: they demand strictly supervised tests in controlled conditions. And it has a point. With many surfers likely to be burning the midnight oil or fending off children and other distractions, concentration levels are bound to fluctuate - and so are scores. The Net also offers ample scope for cheating.

Cautions about the limitations of the tests and reassurance for low scorers are often inadequate. The BBC recently enticed 90,000 people to complete its Test the Nation IQ quiz.

Its message board is littered with gripes about its validity. Some contributors, who failed to reach the average score of 109, were left worrying about premature dementia or the "Simpson gene" .

These folk should probably steer clear of the Guardian's provocative challenge to see if their salaries match their IQs and, conversely, if their IQs justify their salaries.

The site at http://queendom.com offers an IQ test, with no time limit, aimed at discovering your potential intelligence. Unusually, calculators, pens and paper are permitted, but software is not.

The truth is that after sitting half a dozen online IQ tests, I have learnt only that my IQ lies somewhere between the third and 93rd percentile.

Equipped with this knowledge, my interview should be a breeze.