I’ve taken a few different IQ tests over the years, and the results never seem to match. On one test I got 128, another gave me 118, and an online one said 135. I know not all tests are equal, but what actually causes those differences?
Do tests like the WAIS, Stanford–Binet, and Cattell measure the same kind of intelligence, or do they emphasize different abilities? I’ve also heard that the standard deviation can change between tests, which might shift the score range a bit. Is that part of it too?
You’re exactly right that different tests emphasize different abilities. The WAIS focuses heavily on four or five distinct indices (Verbal Comprehension, Working Memory, etc.), while a test like Cattell’s Culture Fair Test isolates Fluid Reasoning (non-verbal pattern solving) and downplays Crystallized Knowledge (vocabulary). If you are exceptionally strong in fluid reasoning but just average in working memory, a test that weighs fluid reasoning more heavily will give you a higher overall score.
Not so surprised with your score on the online tests since they are… not great. A lot of them aren’t properly normed or validated, so they can be all over the place. Some are way too easy and inflate scores to make people feel good.
But afaik, score differences can come from what each test prioritizes, how they’re scaled, and how well they’re actually designed. If you really want an accurate number, the official tests administered by a psychologist are the way to go, but even then, expect some variation just from normal daily stuff like how tired you are or test anxiety. But the differences between scores shouldn’t be that high.
If you’re good with words but average at puzzles, you’ll score higher on WAIS than Cattell. That’s normal—they test different strengths.
The scoring scale varies: Some tests use different “standard deviations.” A 120 on one scale might equal 132 on another, even though you’re at the same percentile.
Your scores (118, 128, 135): If the 135 was online, it’s probably inflated. Online tests boost scores so you’ll share them.
If all three were legitimate, you’re likely around 120-125. Good tests should put you within 5-7 points of each other.
Even taking the same test twice gives slightly different scores. Why? Your score depends on how tired you are, whether you’re anxious, if you had coffee, and pure luck on some questions. Even the best tests have a ±3-5 point margin of error.
Different tests make it worse because they emphasize different skills (words vs. shapes vs. speed) and compare you to different groups of people.
About your scores: if the online test gave you 135, it’s probably fake or inflated. If professional tests gave you 118 and 128, your real IQ is probably somewhere around 120-125. Think of IQ as a range, not a single number. If you need an accurate score for something important, get one professional test from a licensed psychologist. Otherwise, don’t overthink the differences.