5 - Variability and Standard Scores

5 - Variability and Standard Scores - Chapter 6:...

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Chapter 6: Variability and Standard (z) Scores Measure of Variability Single summary figure describing the spread of observations in the distribution Means different things in different settings Imprecision in a measure Individual differences “Error” in a study
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Measures of variability are quantitative indices of the extent to which scores in the distribution are similar or dissimilar Not Information about an individual score Shape of the distribution
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How would you describe the variability (spread) of scores?
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Variability measures include Range Semi-Interquartile Range Average Absolute Deviation Variance Standard Deviation
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Range Range Difference between the lowest and highest score in the distribution A distance (like other variability measures) Central tendency is location For grouped data, use highest and lowest possible raw scores
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Range Only two scores affect the value Highly unstable Tends to increase with sample size Little use in inferential statistics
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Semi-Interquartile Range Semi-Interquartile Range (Q) Half the distance between the first and third quartiles (i.e., P 25 and P 75 ) Quartile Points : The three score points that divide distribution into four parts with = number of frequencies (2nd quartile = P 50 ) Q P P = - 75 25 2
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Semi-Interquartile Range Could also think of Q as mean distance between the median (P50) and the other two quartiles (P25, P75)
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This note was uploaded on 04/17/2008 for the course PSY 201 taught by Professor Arthur during the Spring '08 term at Purdue University-West Lafayette.

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5 - Variability and Standard Scores - Chapter 6:...

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