It's been said that IQ tests measure IQ, where IQ is defined as "that quantity which is measured by IQ tests". The interesting part is that, even defined in th

It's been said that IQ tests measure IQ, where IQ is defined as "that quantity which is measured by IQ tests". The interesting part is that, even defined in that rather circular manner, IQ can be consistently measured and compared between individuals and has strong, reliable correlations with performance on other tasks commonly seen as intelligence-based, as well as correlating with both academic and professional success. Yes, these are correlations, not guarantees, but don't underestimate the value of that, and it's not actually clear that there's any better way. The unknown quantity that such tests are trying to measure is usually referred to as g, or the general intelligence factor. Look up the term "g-loaded" for more information.