Statistics Ch. 11
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repeated-measures design/ within-subject design
a single sample of individuals is maeasured more than once on the same dependent variable. The same subjects are used in all of the treatment conditions.
each individual in one sample is matched with an individual in the other sample. The matching is done so that the two individuals are equivalent (or nearly equivalent) with respect to a specific variable that the researcher would like to control.
For a research study comparing two treatment conditions, what characteristic defferentiates a repeated-measures design from an independent-measures design?
For a repeated-measures design, the same group of individuals is tested in both of the treatments. An independent-measures design uses a separate group for each treatment.
Describe the data used to compute the sample mean and the sample variance for the repeated-measures t statistic.
The two scores obtained for each individual are used to compute a difference score. The sample of difference scores is used to compute the mean and variance
In words and in symbols, what is the null hypothesis for a repeated-measures t test?
The null hypothesis states that for the general population, the average difference between the two conditions is zero. In symbols, uD=0.
What assumptions must be satisfied for repeated-measures t tests to be valid?
The observations within a treatment are independent. The population distribution of D scores is assumed to be normal
Describe some situations for which a repeated-measures design is well suited.
The repeated-measures design is suited to situations in which a particular type of subject is not readily available for study. This design is helpful because it uses fewer subjects (only on sample is needed). Certain questions are addressed more adequately by a repeated-measures design -for example, anytime one would like to study changes across time in the same individuals. Also, when individual differences are large, a repeated-measures design is helpful because it reduces the amount of this type of error in the statistical analysis.
How is a matched-subjects design similar to a repeated-measures design? How do they differ?
They are similar in that the role of individual differences in the experiment is reduced. They differ in that there are two samples in a matched-subjects design and only one in a repeated-measures study.
The data from a research study consist of 10 scores in each of two different treatment conditions. How many individual subjects would be needed to produce these data:
a. For an independent-measures design?
b. For a repeated-measures design?
c. For a matched-subjects design?
- a. The independent-measures design would require 20 subjects (two separate samples with n-10 in each).
- b. The repeated-measures design would require 10 subjects (the same 10 individuals are measured in both treatments).
- c. The matched-subjects design would require 20 subjects (10 matched pairs).
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