what is the difference between reliability and validity in what respects are they similar The real difference between reliability and validity is mostly a matter of definition. Reliability estimates the consistency of your measurement, or more simply the degree to which an instrument measures the same way each time it is used in under the same conditions with the same subjects. Validity, on the other hand, involves the degree to which you are measuring what you are supposed to, more simply, the accuracy of your measurement. It is my belief that validity is more important than reliability because if an instrument does not accurately measure what it is supposed to, there is no reason to use it even if it measures consistently (reliably). Reliability & Validity
We often think of reliability and validity as separate ideas but, in fact, they're related to each other. This is the two ways you can think about their relationship. The metaphors for the relationship between reliability are that of the target. Think of the center of the target as the concept that you are trying to measure. Imagine that for each person you are measuring, you are taking a shot at the target. If you measure the concept perfectly for a person, you are hitting the center of the target. If you don't you are missing the center. The more you are off for that person, the further you are from the center.
The figure above shows four possible situations. In the first one you are hitting the target consistently but you are missing the center of the target. That is, you are consistently and systematically measuring the wrong value for all respondents. This measure is reliable, but no valid that is, it's consistent but wrong. The second shows hits that are randomly spread across the target. You seldom hit the center of the target but, on average you are getting the right answer for the group but not very well for individuals. In this case, you get a valid group estimate, but you are inconsistent....
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