Measurement processes may be biased due to human error. For example, error can result from our visual perceptions -- the difference between what we think we see and the objective reality of what we've actually observed.
The Interactive Activity is based on a visual illusion known as the Muller-Lyer illusion, a phenomenon thoroughly studied by behavioral scientists. In this illusion, two arrows with different arrowheads -- one pointing out and one pointing in -- are placed next to each other. Although the shafts are of equal lengths, the arrow with the outgoing head looks longer than the other arrow.
If you are working with a group, recording the responses of each group member will make the bias demonstrated in the activity more clear. There are two ways you might record results. The simplest would be to record "+" if a person overstates the length and "-" if he or she understates it. Your results should show an obvious difference between pluses and minuses. With a little more effort you could measure the deviation of perceived length and actual length (e.g., by using a tape measure on your monitor) and then look at the average deviation as well as pluses and minuses.
In the second part of the Interactive Activity, you are asked to judge the length of two lines with vertical bars at each end. Judgements in this case tend to be closer to the actual length; any errors are "random."
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