PSYC209 Lecture 9: Testing Means - Two independent-sample t Test with confidence interval

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Instructor Amey Rachel
Textbook Essential Statistics for the Behavioral Sciences by Gregory Privitera
Between-subjects design
- A research design in which different participants are observed one time in each group
or at each level of one factor
- Two ways to do this
o Observe the same participants twice
o Select two different groups
Independent sample
- A type of sample in which different participants are independently observed one time in
each group
- Two ways to select two independent samples
o Select a sample from a population
Used to conduct quasi-experiments
o Select independent samples
Select one sample from the same population and randomly assign
participants in the sample to groups
Used in experiments that include randomization, manipulation and a
comparison/control group
Two-independent sample t test
- A statistical procedure used to compare the mean difference between two independent
groups
- This test is specifically used to test hypotheses concerning the difference between two
population means, where the variance in one or both populations is unknown
We make four assumptions for a two-independent sample t test
- Normality
o We assume the data in each population being sample are normally distributed
- Random sampling
o We assume that the data we measure were obtained from samples that were
selected using a random sampling procedure
- Independence
o We assume that each measured outcome or observation is independent,
meaning that one outcome does not influence another
- Equal variance
o We assume that the variances in each population are equal to each other
o This assumption is usually satisfied when the larger sample variance is not
greater than two times the smaller
Two ways to state that there is no difference between two population means
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