Sigall and Ostrov (1975) did an experiment to assess whether the physical attractiveness of a defendant on trial for a crime had an effect on the severity of the sentence given in mock jury trials. Each of the participants in this study was randomly assigned to one of the following three treatment groups; every participant received a packet that described a burglary and gave background information about the accused person. The three treatment groups differed in the type of information they were given about the accused person's appearance. Members of group 1 were shown a photograph of an attractive person; members of group 2 were shown a photograph of an unattractive person; members of group 3 saw no photograph. Parts of their results are described here. Each participant was asked to assign a sentence (in years) to the accused person; the researchers predicted that more attractive persons would receive shorter sentences.
a. Prior to assessment of the outcome, the researchers did a manipulation check. Members of group 1 and 2 rated the attractiveness (on a 1 to 9 scale with 9 most attractive) of the person in the photo. They reported that for the attractive photo, M = 7.53; for the unattractive photo, M = 3.20, F(1,108) = 184.29.
a. Was this difference statistically significant (using α = .05)?
b. What was the effect size for the difference in 2a?
c. Was their attempt to manipulate perceived attractiveness successful?
d. Why does the F ratio in 2a have just 1 df in the numerator?
e. The mean length of sentence given in the three groups was as follows:
Group 1, attactive photo, M = 2.80
Group 2, unattractive photo, M = 5.20
Group 3 , no photo, M = 5.10
They did not report a single overall F comparing all three groups; instead they reported selected pair wise comparisons. For Group 1 versus Group 2, F(1,108) = 6.60, p < .025.
Was this difference statistically significant? If they had done an overall F to assess the significance of differences of means among all 3 groups, do you think this overall F would have been statistically significant?
f. Was the difference in mean length of sentence in part 2e in the predicted direction?
g. Calculate and interpret an effect size estimate for this obtained F
h. What additional information would you need about this data to do a Tukey HSD test to see whether Groups 2 and 3, and 1 and 3, differed significantly?