Lets explore main effects and interactions

1
Lets explore main effects and interactions!

• Andrea M. Karkowski
• Behavioral Sciences Department
• Capital University
• Columbus, OH 43209

2
Whats a Main Effect

• When one IV has an effect on the DV (ignoring all
  other IVs), then you have a main effect. That is,
  peoples behavior is different at different
  levels of the IV.
• Example with two IVs
• Your variables
• IV1 Setting (bar or private party)
• IV2 Amount of Alcohol (0 oz, 12 oz, 24 oz)
• DV How foolish a person is when flirting with
  someone
• If foolishness changes with amount of alcohol,
  regardless of setting, then you have a main
  effect for alcohol.
• If foolishness changes with setting (e.g.,
  people are more foolish in a private party than
  at a bar), regardless of how much they have had
  to drink, then you have a main effect for setting.

3
Whats an Interaction

• When the effect of one IV changes with different
  levels of a second IV.
• Using the example from the previous slide
• If foolishness changes with the setting, and
  those changes are differentially affected by how
  much a person has had to drink, then you have an
  interaction between setting and amount of
  alcohol.
• In the graph, people are really foolish at
  private parties, regardless of how much they have
  had to drink. BUT, when at a bar, people dont
  become foolish until they have been drinking.

4
Other Possibilities for Main Effects and
InteractionNo Main Effects, No Interaction
5
Other Possibilities for Main Effects and
InteractionMain Effect for Self-esteem, No
Interaction
6
Other Possibilities for Main Effects and
InteractionMain Effect for Alcohol, No
Interaction
7
Other Possibilities for Main Effects and
InteractionTwo Main Effects, No Interaction
8
Other Possibilities for Main Effects and
InteractionNo Main Effects, Interaction
9
Other Possibilities for Main Effects and
InteractionTwo Main Effects, Interaction
10
Test YourselfAre there Main Effects and
Interactions?
11
No M.E. for Self-esteem, No M.E. for
AlcoholInteraction between Self-esteem and
Alcohol
12
Test YourselfAre there Main Effects and
Interactions?
13
M.E. for Self-esteem, M.E. for AlcoholInteraction
between Self-esteem and Alcohol
14
Test YourselfAre there Main Effects and
Interactions?
15
M.E. for Self-esteem, No M.E. for Alcohol No
Interaction
16
Test YourselfAre there Main Effects and
Interactions?
17
M.E. for Self-esteem, M.E. for Alcohol
Interaction between Self-esteem and Alcohol
18
Test YourselfAre there Main Effects and
Interactions?
19
M.E. for Self-esteem, No M.E. for
AlcoholInteraction between Self-esteem and
Alcohol
20
?????
How did you do?