University of Cambridge Cognitive Ability Test

"You achieved a total score of 50! This means that you are 70% higher than 13351 participants who have completed the test. The percentile score is calculated on-the-fly. This means that your percentile rank may change as more people complete the test."

Not sure this was worth 65 minutes of my time. Would have liked to see whether whatever this version of the ICAR60 is is pegged to a standardized (IQ) test score. I'm assuming the 13,000 people who have also taken this are not representative of the wider pop.

The self-selection bias is definitely something to consider. I’d guess people who feel relatively confident in their intelligence are more likely to take the test.

Many highly intelligent people might decide that it is not worth their time though.

True, and many not so intelligent people may overestimate their intelligence and do it. So maybe it does somehow end up being representative.