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3/11/12 Standard deviation
1/3une.edu.au/WebStat/unit_materials/c4_descriptive_statistics/standard_deviation.htm
� Chapter 4: Analysing the DataPart II : Descriptive Statistics
� Standard deviationTo overcome the problem of dealing with squared units,statisticians take the square root of the variance to get thestandard deviation.
The standard deviation (for a sample) is defined symbolicallyas
So if the scores in the data were 5, 7, 6, 1, and 8, theirsquared differences from the mean would be 0.16 (from [55.4]2), 2.56 (from [75.4]2), 0.36 (from [65.4]2), 19.36 (from[15.4]2), and 6.76 (from [85.4]2). The mean of these squareddeviations is 5.84 and its square root is 2.41 (if dividing by N),which is the standard deviation of these scores. The standarddeviation is defined as the average amount by whichscores in a distribution differ from the mean, ignoring thesign of the difference. Sometimes, the standard deviation isdefined as the average distance between any score in adistribution and the mean of the distribution.
The above formula is the definition for a sample standarddeviation. To calculate the standard deviation for apopulation, N is used in the denominator instead of N1.Suffice it to say that in most contexts, regardless of thepurpose of your data analysis, computer programs will printthe result from the sample sd. So we will use the second
3/11/12 Standard deviation
2/3une.edu.au/WebStat/unit_materials/c4_descriptive_statistics/standard_deviation.htm
formula as our definitional formula for the standarddeviation, even though conceptually dividing by N makesmore sense (i.e., dividing by how many scores there are to getthe average). When N is fairly large, the difference betweenthe different formulas is small and trivial. Using the N1version of the formula, we still define the standard deviation asthe average amount by which scores in a distribution differfrom the mean, ignoring the sign of the difference, eventhough this isn�t a true average using this formula.
The standard deviation in our sexual behaviour data is 2.196,from the SPSS printout in Output 4.2. So the mean number ofsexual partners is 1.864 with a standard deviation of 2.196.The units are now the same as the original data. But, is this alarge standard deviation? It is hard to say. In a normaldistribution the mean and standard deviation are independentof each other. That is one could be large or small and theother large or small without any influence on each other.However, in reality they are often linked so that larger, meanstend to have larger standard deviations. This leads into thearea of transformations that are a way of reestablishing thisindependence.
A useful measure of a distribution that is sometimes used isthe ratio of the standard deviation to the mean (Howell p. 48)
The standard deviation has one undesirable feature. Like themean, one or two extreme scores easily influence thestandard deviation. So really atypical scores in a distribution("outliers") can wildly change the distribution�s standarddeviation. Here, adding a score of 200 increases the sd from2.196 to 15.0115, a sevenfold increase! Because both ofthese descriptive statistics are influenced by extreme cases, itis important to note when extreme values exist in your dataand might be influencing your statistics. How to define"extreme," and what to do if you have extreme data points is acontroversial and complex topic out of the scope of this class.
3/11/12 Standard deviation
3/3une.edu.au/WebStat/unit_materials/c4_descriptive_statistics/standard_deviation.htm
�
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