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Lecture 2

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01/18/2016 Lecture 2

Paired T Test/Repeated Measures: used when comparing the scores of the same group of individuals

(tested at two different times)

Null hypothesis = H0: µdiff = 0 (population mean difference = 0)

oAssumes the sample is taken from a population with a mean difference of 0

oAny difference between the mean in the sample and the hypothesized population mean

of 0 is simply due to random sampling error

Alternative hypothesis = H1: µdiff does not equal 0 (population mean difference is less/greater

than 0)

oIf you reject the null hypothesis you conclude that the alternative hypothesis is correct

oThis means that the difference between the sample mean and the hypothesized

population mean is so large that it could not be due to random sampling

Assumptions:

1. Random sampling from a clearly defined population

2. Population difference values are normally distributed

There is no Levene’s test since the same people are being used in both groups

APA Results Section Example

Effect Size

Describes the magnitude of differences using Cohen’s d

D = Mgroup1 – Mgroup2 / SDpooled (OR SDs separate if variances are not equal)

Small effect = .20

Medium effect = .50

Large effect = .80

Choose the guideline that is closest to the value in your calculation

P-values do not tell you about the size of the effect

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