Reliability denotes the consistency or repeatability of a result of a test in case a similar test is done again or for a second time. A test is said to be reliable if the result in the two tests are similar. In case the two tests give varying results, then the test is not reliable. This is normally based on the same testing environment to ensure that the result do not change due to change of environmental factors.
A test with very poor reliability demonstrates a great variation of the result. The different forms of reliability include test-retest reliability, internal consistency reliability, parallel or alternate form reliability, and inter-rater reliability.
Reliability can be improved by ensuring clear conceptualization to ensure that the true meaning is obtained, by increasing the items number, through standardization, by using many indicators, using more exact measurements, and through replication and pilot testing (Miami Psych,2011b).
Validity denotes the believability or credibility of a test. It determines the accuracy of the test and whether the rests are genuine. Validity can be evaluated by checking on whether the right test has been used, following the right procedures, and maintaining the required standards. The different types of validity include content validity, face validity, discriminant validity, criterion validity, convergent validity, and construct validity.
Validity can be improved by using the right equipment with the right items and that are constructed correctly or calibrated effectively to ensure correct measurements. Validity can also be improved by enlarging the sample to ensure that the result can stand for a more extensive population. Validity can as well be improved by investigating the instrument using double-blind or single-blind study method (Miami Psych, 2011a).