Many students get a little muddled initially by these two concepts and so it may be helpful, at first, to suggest that they avoid using them in the same sentence. That is, try to avoid, “the study lacked reliability and validity”. Perhaps it did, but what is more likely is that it lacked one of these. Reliability can be thought of as repeatability – the extent to which, if you repeated the research, you would get the same results. Validity is whether you are measuring what you say you were measuring.
A good example of how to illustrate this in a lesson is using fabric tape measures (perhaps the ones you can find in that well-known Swedish furniture store!) Instruct students to work in pairs and to measure the circumference of each other’s head. Once they have made a note of this tell them that this is a measure of intelligence – the larger the head, the more intelligence. Clearly this is not the case and discussions will ensue. From this you can introduce the term validity – you are not measuring intelligence, you are measuring head size. Then ask them what will happen if they repeat the measurement (and get them to do this). The measurement will be the same, allowing for a small margin of change for positioning of the tape measure). The experiment has been repeated with the same results. So, there is high reliability but low validity.
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