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Chapter 17 Descriptive Statistics Part 2 (Measures of Variability (how…
Chapter 17 Descriptive Statistics Part 2
Skewness
Positively skewed
tail on the right
e.g. difficult test
mean > median > mode
Negatively skewed
tail on the left
e.g. easy test
mean < median < mode
Measures of Variability (how spread out the data values are)
Range
H-L
limited - severely affected- extreme numbers
Variance
average deviation from mean - squared units
Standard Deviation
square root of variance
how far - numbers - vary - from mean
Measures of Relative Standing (where a score falls in comparison to other scores in the data)
percentile ranks
% of scores - reference group - falls below - a raw score
z- scores
raw score -> standard dev. units
how many std devs - raw score- from the mean
raw score above mean : + z score
raw score = mean : 0 z score
can combine z-scores and percentile ranks if normally distributed
Examining r/s among variables
Contingency Tables
calculated down, compare across
calculated across, compare down
to see r/s between variables
Regression
Simple
predict the values of DV based on 1 IV
Multiple
predict - values of DV based on 1 / more IV
Y intercept
where the line crosses Y axis
Regression Co-efficient
predicted change in Y per unit change in X
slope
Both variance & std dev will be bigger if the data are spread out