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Chapters_8_9_-_Between_Within_Groups

Chapters_8_9_-_Between_Within_Groups - PSYC 310 Chapters 8...

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Between & Within Subjects Designs Between & Within Subjects Designs PSYC 310 – Chapters 8 & 9 PSYC 310 – Chapters 8 & 9
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Between Groups Design Between Groups Design Two or more groups are formed at random from a pool of subjects. (independent groups) Each group receives a different experimental treatment (value of the IV) and the groups are compared. Group 1 Group 2 Results Results
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Between Groups Design Between Groups Design Because participants experience only one level of the IV, you end up with 1 score per participant. These are independent measures. Group 1 Group 2 Music No Music 14 12 18 16 15 10 11 9 7 10
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Between Groups Design Between Groups Design The difference in the DV between groups is referred to as SYSTEMATIC VARIANCE. Scores also vary within groups (individual differences occurring by chance) NON-SYSTEMATIC VARIANCE. To look for an effect of your IV, you compare the mean scores (DV) for each group. Non-systematic variance is an important source of error and therefore must be minimized.
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Between Groups Design Between Groups Design To determine statistical significance, you compare between groups variance to within groups variance. F = between-group variance (systematic) within-group variance (error) When there is a large within-group variance, it is difficult to see an effect – want to minimize it. Large BG variance is good; large WG variance is bad.
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Between Groups Design Between Groups Design For groups to be equivalent they must be: - created equally: random assignment - treated equally: blind study - composed of equivalent individuals: make sure you have similar age groups, etc… To keep with-in group variance low, you need to ensure that your groups are equivalent.
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Between Groups Design Between Groups Design Randomization Most powerful technique to control for the effect of pre-existing differences by equalizing them (spreading them evenly). Participants are randomly assigned to groups to ensure groups are as equal as possible before treatment.
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Between Groups Design Between Groups Design Be careful! Randomization is not the same as random sampling. RS - random selection of Ss from a larger population to participate in a study. RA - random assignment of Ss to experimental or control groups in a particular study.
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Randomization: different methods Randomization: different methods 1. Free Random Assignment : If more than 2 groups, a table of random numbers is used to guard against the bias of repetition.
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