The honest answer is: less accurate than a supervised assessment, more accurate than nothing, and wildly variable between sites. The difference between a useful online test and a decorated quiz has little to do with how the site looks and a lot to do with what it is willing to tell you.
What limits accuracy online
- Nobody is supervising: interruptions, second attempts and looking things up are all possible.
- Conditions are unknown — sleep, noise, screen size, whether you were rushing.
- Most online banks are small, so familiarity from a previous attempt inflates later scores.
- Self-selected visitors are not a representative population sample, so norms built purely on site traffic drift.
Warning signs a test is theatre
- A single confident number with no margin of error.
- A score you only see after paying, or after handing over an email.
- Suspiciously flattering results — sites that tell nearly everyone they are gifted are optimising for sharing, not measurement.
- Claims of accreditation, or borrowed authority from Mensa, universities or test publishers.
- No explanation of how the score was produced, and no way to see your own answers.
Signals a test is being straight with you
- Results reported as a range with an explicit margin of error.
- A stated methodology, including what is inherited from published research and what is not.
- Plain limitations: what the result cannot be used for.
- Your result visible without payment, with paid options adding precision or depth rather than unlocking the number itself.
- Some fraud handling — runs answered impossibly fast, or with the same option every time, should be thrown out rather than scored.
So how close can an online result get?
Under decent conditions, a well-calibrated online reasoning test can usually place you in the right broad band — the right side of average, roughly which quarter of the distribution. It should not be expected to pin you to a specific number, and any site that claims otherwise is claiming precision it cannot have. If you need a result for a diagnosis or any formal decision, an online test is the wrong instrument entirely; see a licensed psychologist.