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Dependent Samples tTest
Practice Problems
2
Recap of Last Class
• A research design that involves
dependent samples
is one
where the observations from one sample are related in
some way to those from the other.
• Before vs after (repeated measures)
• Or two matched samples.
• Essentially the same as the singlesample ttest. One
major distinction is that it is based on difference scores (D)
rather than raw scores (X scores).
• The sign of each D score tells you the direction of the
change.
The sample of difference scores will serve as the
sample data for the hypothesis test.
3
Recap of Last Class
• Interested in knowing what would happen if every
individual in the population were measured in two
treatment conditions (X
1
and X
2
) and a difference score
were computed for every one.
• The null hypothesis states that there is no effect, no
change or no difference.
• Specifically interested in the mean of the population of
difference scores (M
D
). Some will show positive difference
scores and some will show negative scores, but the
differences will balance out to 0.
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Recap of Last Class
• The variance and standard deviation of the population of
difference scores are estimated from the sample difference
scores.
• The distribution of means of difference scores has a mean
of 0, the same as the mean of the population of difference
scores.
• The standard deviation of the distributions of means of
difference scores (i.e., estimated standard error) is the
estimated population standard deviation of difference
scores divided by the square root of the sample size.
5
Steps of 2sample Dependent Means
• Step 1:
Formulate Hypotheses
• Step 2:
Sample distribution
• Step 3:
Calculate appropriate statistics
• Step 4:
Determine critical cutoff
 Need alpha
 1 or 2 tailed depending on question
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This note was uploaded on 04/29/2010 for the course PSYCH 315 taught by Professor Afroditipanagopoulos during the Winter '10 term at Concordia Canada.
 Winter '10
 AfroditiPanagopoulos

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