What is the relationship between reliability and validity?
Validity is whether or not you are measuring what you are supposed to be measuring, and reliability is whether or not your results are consistent.
If an instrument or experiment is valid, it will usually also be reliable as long as it is carefully constructed to control all variables except the one being studied.
“For a test to be valid, or truthful, it must first be reliable.” (educational assessment)
For example, if you are measuring the reading levels of students after a new computer-based reading program has been used, you will want to make sure that your test will get the same results every time it is taken, and it should if it accurately measures the students’ reading ability.
If an instrument or experiment is reliable, it does not necessarily have to be valid. You might get consistent results, but not actually be measuring what you think you are measuring. For example, if the above test of reading level was really a vocabulary test, you might get the same results each time but the results do not show reading level.
Reliability is the ability of a test to yield the same findings or results each time it is conducted. A test that yields different results every time it is conducted with everything else being constant cannot be said to be a reliable test or study. Thus, reliability speaks to the consistency of the test or study, which means no variations in the observed results or findings.
Validity is the credibility of the study or the genuineness of the study. Results of a test should be correct by ensuring that what is being measured and the tools used to measure are what is required. Additionally, validity also refers to the ability of the results to be generalized to the study population. Thus, the results should apply to the same subjects beyond the study.
The relationship between the two is that if the study is valid, then it must be reliable. However, a study's reliability does not automatically attest to the validity of the study. Although necessary, reliability is not in itself a condition for validity.