What are extreme scores? They are scores far outside the
norm for a variable or population, leading to the conclusion that they are not
part of your true population and probably do not belong in your analyses. A
common operationalizing definition for extreme scores is +/-3 standard
deviations (SDs) from the mean.
Recall that standard normal distribution of a population has
68.26% of the population between +1 and -1 SD of the mean (see attached
diagram: 34.13% between 0 to +1 SD +
34.13% between 0 and -1 SD = 68.26%).
So 95.44% of the population should fall between 2 SD from
the mean (34.13% + 34.13% + 13.59% +13.59% = 95.44%), and 99.74% of the
population should fall 3 SD of the mean. In other words, the probability of
randomly sampling an individual more than 3 SD from the mean in a normally
distributed population is 0.26%, which gives good justification for considering scores
outside 3 SD as suspect. Our concern is that these scores are not part of the
population of interest in your study.
Next time I will post an updated blog index. Do you have an
issue or a question that you would like me to discuss in a future post? Would
you like to be a guest writer? Send me your ideas!
leann.stadtlander@waldenu.edu
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