# What is standard deviation?

I know it is a measure of dispersion but I don't understand the questions that come up on it.. I don't understand why a low mean is better than a high mean? I'm not really sure on it if someone is willing to help.. thanks

Posted Wed 19th December, 2012 @ 22:34 by ax5za

## 2 Answers

In my resources I have made a word document explanaining how to do it and what it is. It's for biology but I assume you do the same thing.

Here is some more info about standard deviation from the website: http://wiki.answers.com/Q/What_does_it_mean_if_the_standard_deviation_is_greater_than_the_mean

The standard deviation and the arithmetic mean measure two different characteristics of a set of data. The standard deviation measures how spread out the data is, whereas the arithmetic mean measures where the data is centered. Because of this, there is no particular relation that must be satisfied because the standard deviation is greater than the mean.

Actually, there IS a relationship between the mean and standard deviation. A high (large) standard deviation indicates a wide range of scores = a great deal of variance. Generally speaking, the greater the range of scores, the less representative the mean becomes (if we are using "mean" to indicate "normal"). For example, consider the following example:

10 students are given a test that is worth 100 points. Only 1 student gets a 100, 2 students receive a zero, and the remaining 7 students get a score of 50.

(Arithmetic mean) = 100 + 0(2) + 7(50) = 100 + 0 + 350 = 450/10 students
SCORE = 45

In statistics, the median refers to the value at the 50% percentile. That means that half of the scores fall below the median & the other half are above the median. Using the example above, the scores are: 0, 0, 50, 50, (50, 50), 50, 50, 50, 100. The median is the score that has the same number of occurrences above it and below it. For an odd number of scores, there is exactly one in the middle, and that would be the median. Using this example, we have an even number of scores, so the "middle 2" scores are averaged for the median value. These "middle" scores are bracketed by parenthesis in the list, and in this case are both equal to 50 (which average to 50, so the median is 50). In this case, the standard deviation of these scores is 26.9, which indicates a fairly wide "spread" of the numbers. For a "normal" distribution, most of the scores should center around the same value (in this case 50, which is also known as the "mode" - or the score that occurs most frequently) & as you move towards the extremes (very high or very low values), there should be fewer scores.

Answered Thu 20th December, 2012 @ 11:47 by Former Member

Thank you so much.. very helpful :)

Answered Fri 21st December, 2012 @ 23:38 by ax5za