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PostHoc Comparisons. Most liberal/conservativce?
PostHoc comparisons are comparisons run after data collection.
Number of possible comparisons c = (a)*(a1) with a=# of groups. To determine pairwise comparisons just divide by 2
Fisher’s LSD (very liberal) > Modified Bonferroni> Bonferroni> Tukey’s HSD (very conservative)

Familywise Error
The probability of at least one Type I error (a) for a set of many comparisons. aFW = (c)(a) with c= # of comparisons

“Fishing”
Conducting a large number of statistical tests, without theoretical reasons, and then “fishing” out the significant test results.

Calculating Fisher’s LSD
 Anything greater than Fisher's LSD is significant.

T=
t=
A=
a=
AS=
s=
as=
[T]=
[A]=
[AS]=
 T= Grand Total
 t= # of grand totals (always 1)
 A= Total for each condition
 a= # of levels for the IV
 AS= Individual scores
 s= # of participants per condition
 as= Total # of participants
 [T]=T^{2}/as
 [A]=(A_{1}^{2}^{}+A_{2}^{2})/s
 [AS]=AS^{2}

Planned Comparisons
 Planned before the study is executed to test a specific hypothesis.
 Only difference in how they are conducted is the alpha level used to determine significance.

Planned Comparisons: Rules for Creating Coefficients
 There must be 1 coefficient for each mean
 – Means not included in the comparison have a coefficient of zero
 There is always two sets of means being compared
 – means in the same set have the same sign (+ or )
Coefficients must add to zero

Orthogonality
Orthogonal comparisons reflect independent or nonoverlapping pieces of information.
The outcome of one comparison gives no indication whatsoever about the outcome of another orthogonal comparison.
Nonorthogonal contrasts can be hard to interpret.

What are the advantages of a WithinSubjects Design?
 Control of individual differences (aka subject differences)
 More likely to find an effect
 Fewer participants needed
 Efficiency gains (less timeconsuming and expensive)…
 Sequence effects
 2) Carryover effects
 a) Order
 b) Practice
 c) Interference
 d) Differential

What confounds are specific to a WithinSubjects Design?
 Confounds are extraneous variables that vary in some nonrandom way across conditions
 1. Sequence
 :– People change as they spend time working on tasks
 – They can become bored, fatigued, or more excited.
 2. Carryover:
 – Responses to one condition influence the responses to a later condition
 .– Different from sequence effects. Not the passage of time, but the impact of one condition on another.
 2) Carryover effects
 a) Order
 b) Practice
 c) Interference
 d) Differential

