Results of numerous psychological tests (primarily intelligence tests) are most commonly expressed in IQ-scores. These are constructed in such a way that the mean population score equals 100 andthe standard deviation equals 15 by definition. Thus, the majority of people (around 68%) achieves an IQ-score in the range of 100 ± 15, if the measured variable follows an approximately normal distribution. In fact, any quantitative variable, e.g. your body height, can be converted to the IQ-scale, as you can try by using this application and answering a few questions.
However, no psychological test is perfectly reliable. Consequently, it is likely that the measured value will deviate from the "true" IQ. This difference between a measured and a true value is called measurement error. However, if we knew your actual height and test reliability, we could simulate the variability of the measured values in repeated testing despite your height remaining the same. Using the following app, you can see yourself how (in)accurate psychological measurement could be and why we cannot ignore measurement error.
© 2020 Karel Rečka Department of Psychology, FSS MU, Brno, Czech Republic.