# SD Pros and Cons

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16.7
Sometimes we can’t use the mean because we have too much skew or extreme scores. In those situations, the mean is meaningless, no pun intended. Go ahead and look at the example on the screen. Here, we have an extreme score up around 13, and as a result, our average for this set of numbers is above 4. Of course, the typical score is around 2 or 3, so clearly, the mean here is biassed or not useful. Well, the standard deviation is based on the mean. Remember? We find how far people differ from the mean. And in this case, we wouldn’t use the standard deviation either.
52.5
So as a general rule, when you can’t use the mean because of skew, you also can’t use the standard deviation. Shares all the same strengths and weaknesses of the mean. So in this case, we’re not going to use the standard deviation. We’re going to have to find an alternative measure. So in the next lesson, we’re going to develop an alternative or a backup to the standard deviation, just like we had to do for our averages.