es (Marginal Mean=3.33). Second, those with a positive genetic family history of substance abuse (Marginal Mean = 8.25) were significantly more depressed than those with a negative family history of substance abuse (Marginal Mean = 2.95).
The next analysis performed was another two-way between-subjects analysis of variance; this analysis was conducted to examine whether study participants' MacAndrews scale scores (the measure of substance usage) significantly differed as a function of differences in Adoption Status and/or Genetic History of substance abuse variables. The findings observed for this analysis are presented, along with cell and marginal means, in Table 4.2
As can be seen from examination of this table, two findings were significant: (1) a main effect for the Adoption Status variable (F = 8.384; p<.01); and (2) a significant Adoption Status by Genetic Family History of substance abuse interaction (F = 5.481; p<.05).
As noted by Linton and Gallo (1975), interpretation of the findings of a two-way analysis of variance, in the presence of both a significant main effect or effects and an interaction, is given by the nature of the observed interaction; in other words, the interpretation of the interaction overrides the interpretation of main effects. With respect to the measure of substance abuse, the observed interaction appears to be both interesting and complex.
Analysis of Variance Summary Table: Substance Abuse Data
SOURCE df MS F p
-----------------------------------------------------------------
Adoption Status 1 133.929 8.384 <.01
Genetic History 1 27.125 1.698 n.s.
-----------------------------------------------------------------
GENETIC HISTORY OF SUBSTANCE ABUSE
ADOPTION STATUS Positive Negative Marginal
...