When tests are performed on the same subject multiple times as a method of determining reliability, then reliability becomes related to the time intervals between successive testing. In general, shorter time intervals lead to higher levels of reliability. In other words, if a researcher is concerned about having reliable results, she should administer retests without too long of waits in between testing runs.
The heterogeneity of a population can lead to a reduction in assessment reliability. This is due to the higher level of variance associated with diverse groups. Typically, groups that are homogeneous on traits related to the assessment's quality of interest tend to produce more reliable results. Researchers chasing highly reliable results should then attempt to find a homogeneous group to play the role of the study's sample.
Well-designed questions can increase a test's reliability. The key technique in question design that boosts an assessment's reliability is objectifying the questions in an unambiguous way. If the questions are easily interpreted in multiple ways, the reliability will suffer, as when individuals retest, they are likely to interpret certain questions differently each time. Hence, researchers should make efforts to assure that their test items will not confuse the test takers.
Random error has a large impact on any form of assessment. Its impact on an assessment's reliability also is well-founded. The reduction of random error always leads to a higher reliability, because it reduces the variance associated with retesting as well as within-test items. It is not easy to reduce random error in testing and assessments, but researchers can use calibration techniques and pilot studies to find which questions are not suitable or should be modified. By tweaking and fine-tuning the items on an assessment, researchers can increase the reliability of the assessment without having to limit themselves to specific populations or time intervals.