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Lecture 4

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University of Toronto Scarborough
Anthony Ruocco

Lecture 4 January 31 2012 Measures of Variability Standard deviationconceptually it is the degree of dispersion in a group of scores Preferred measure of variability because of its direct relevance to the normal distribution Variancesquared value of the standard Deviation Figure 35 Skewed Distribution CurvesSkewness is one measure we use to tell us about the symmetry or asymmetry about the distribution One stretching to the left is lower scores negative skewOne stretching to the right is higher scorespositive skew When scores are at low end positive skew the test probably contains too few easy items If scores are massed at the high end negative skew the test probably contains too few hard itemsAgreeableness Normally DistributedPeople are higher than average in reporting agreeablenessRaw Score TransformationsThe most basic level of information provided by a psychological test is the raw scoreTo be AT the first percentile is not a good thing in Psychology it means you are at the very bottom at the scoreth You want to be at the 99 percentile this means your score is better than 99 of the people Tscores and other standardized scoresTscore of 60 means they got through a lot of tasks whereas a Tscore of 20 means they didnt get through a lot of tasksMeasures of Central TendencyCompute the mean by adding all the scores up and diving by N the number of scoresThe median is the middlemost score when all the scores have been rankedMode is the most frequently occurring scoreIf the scores are tightly packed around a central value the SD is small Figure 37
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