Quartile error is the quartile deviation of the expected error; that is, the quartile deviation of an individual's scores if it were possible to take the test repeatedly without a learning effect between the test administrations. A rule of thumb for interpreting quartile error: One's true score on the test in question lies with 50 % probability between plus and minus one quartile error from one's actual score.
Quartile error is computed similarly to standard error, but with the test's raw score quartile deviation instead of its standard deviation. In a normal distribution, the quartile error is equal to the "probable error", which actually is computed as .6745 × σerror .