Validity and Reliability
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| Validity | Reliability |
| Validity implies the extent to which the research instrument measures, what it is intended to measure. | Reliability refers to the degree to which assessment tool produces consistent results, when repeated measurements are made. |
| It refers to the ability of the instrument/test to measure what it is supposed to measure | It refers to the reproducible of the results when repeated measurements are done |
| It relates to the correct applicability of the instrument/test/procedure in a needed situation  | It relates to the extent to which an experiment, test or any procedure gives the same result on repeated trials. |
| Can relate to question of ‘Does it measure what it is supposed to measure’?  | Can relate to question, ‘How representative is the measurement’? |
| Answers, ‘Is it the right instrument/test for what I need to measure?’ | Answers, ‘Can the results obtained be replicated if the test is repeated?’ |
| Validity looks at accuracy | Reliability looks at repeatability/consistency |
| Validity mainly focuses on the outcome | Reliability mainly focuses on maintaining consistent result |
| Influencing factors for validity are: process, purpose, theory matters, logical implications, etc. | Influencing factors for reliability are: test length, test score variability, carcinogenicity, etc. |
| Validity has more analysis and is harder to achieve. | Reliability is comparatively easier and yields faster results. |
| There cannot be validity without reliability    | There can be reliability without validity. |
| Even if validity of an instrument is poor (for certain test), it can have high reliability (for other tests)Â Â Â Â Â Â Â Â | When reliability/repeatability is poor, validity may also be poor. Thus, usefulness of a test/experiments are negligible. |
| If the results are not valid, the test is of no use at all       |
If the results cannot be replicated, the test is of little use |
