Standard error of measurement is the standard deviation of the expected error; that is, the standard 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 standard error: One's true score on the test in question lies with 95 % probability between plus and minus two standard errors from one's actual score.
In interpreting standard error one may also consider that its value really only applies to the middle part of the test's score range, and loses meaning at the edges. In general, standard error is only meaningful where the scale on which it is expressed is linear, which is not everywhere and always the case.
Standard error (σerror) is computed by combining a test's reliability (rxx) with its raw score standard deviation (σ):
σerror = σ × √(1 - rxx)