Compute the estimated Cohen’s d

Many animals, including humans, tend to avoid direct eye contact and even patterns that look like eyes. Some insects, including moths, have evolved eye-spot patterns on their wings to help ward off predators. Scaife (1976) reports a study examining how eye-spot patterns affect the behavior of birds. In the study, the birds were tested in a box with two chambers and were free to move from one chamber to another. In one chamber, two large eye-spots were painted on one wall. The other chamber had plain walls. The researcher recorded the amount of time each bird spent in the plain chamber during a 60-minute session. Suppose the study produced a mean of M = 37 minutes in the plain chamber with SS = 288 for a sample of n = 9 birds. (Note: If the eye-spots have no effect, then the birds should spend an average of

μ = 30 minutes in each chamber.)

a. Is this sample sufficient to conclude that the eyespots have a significant influence on the birds’ behavior? Use a two-tailed test with
α = .05.

b. Compute the estimated Cohen’s d to measure the size of the treatment effect.

c. Construct the 95% confidence interval to estimate the mean amount of time spent on the plain side for the population of birds.


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