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Pairedsample ttest
pairedsample ttest
used to compare 2 means where subjects have been measured twice

examples of when to use a pairedsample ttest
pairedsample ttest
  subject is measured twice on a DV pre and post treatment (aka prepost design)
 subject is measured on a DV at different levels of an IV simultaneously (DV = SAT, IV = math, verbal)
 subject can be more than one person (DV = happiness, IV = spouses (husband, wife))

Assumptions
pairedsample ttest
 Normality
 No Outliers
 Independence of subjects

Assumptions: Normality
pairedsample ttest
 the difference score of a DV (error term) should be normally distributed
 tested using ShapiroWilk

Assumption: when Normality is violated
pairedsample ttest
 a research question can still be answered by using an alternative test that does NOT assume normality
 a nonparametric alternative to the pairedsample ttest is Wilcoxon signed rank test

Assumption: Outliers
pairedsample ttest
 has an undue influence on a statistic
 assumption is checked by examining histogram and QQ plot

Assumption: Independence of Subjects
pairedsample ttest
 design considerdation
 one subject score is not influenced by another subject in study
 cannot be tested using statistics

Degrees of Freedom
pairedsample ttest
df= N1
doesn't depend on the number of scores but rather on the number of subjects

Confidence Interval
pairedsample ttest
 interval estimation
 accounts for uncertainty of error

Disadvantages
pairedsample ttest
Order effect
Carryover effects
Test effects

Disadvantages: Order effect
pairedsample ttest
 when the outcome of the study is affected by the order in which the treatments were prepared.
 Two most common types: carryover and test effects

Order effect: Carryover effects
pairedsample ttest
effects of one treatment do not end prior to the administration of another treatment, so the effects observed in the later treatment include residual effects of the former
ex: testing new pill but not waiting long enough between dosages

Order effect: Testing Effect
pairedsample ttest
fatigue and practice effects
effects due to repeated testing of participants over time
ex: taking the same test twice in a short period of time

Counterbalancing
pairedsample ttest
common solution to an order effect
the order of treatments is often counterbalanced so that some participants get treatment in order but other get a second order so that order effects can be assessed.

Advantages
pairedsample ttest
Reduced error: variance within subjects is typically smaller than between subjects (smaller sampling error, more power)
Number of subjects: pairedsamples ttest requires smaller number of subjects to achieve the same power than the independentsamples ttest

