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Lecture 2

PSYC 2040 Lecture Notes - Lecture 2: Null Hypothesis, Alternative Hypothesis


Department
Psychology
Course Code
PSYC 2040
Professor
David Stanley
Lecture
2

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Psych2040 Research Statistics
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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