Audrey also has a sample of fraternal twin pairs. She gives them the same test of extraversion to the fraternal twins and finds that D = 1.34, s_D = 2.40, and n_D = 15.

a. What is the null hypothesis?

b. What would the α value be for this NHST?

c. Is a one- or a two-tailed hypothesis appropriate for this scenario? Justify your response.

d. What is the alternative hypothesis for this paired-samples t-test?

e. Calculate the degrees of freedom in this example.

f. Find the tcrit value.

g. Calculate the tobs value for the data.

h. Should you reject or retain the null hypothesis?

i. Calculate Cohen’s d for these data, using the equation in Sidebar 9.2.

j. Find a p-value for these data.

k. Interpret the results of the paired-samples t-test. In other words, do identical twins have, on average, the same level of extraversion?

l. Compare these results to the results from identical twins. What does that tell you about the similarities (or differences) in personalities among identical and fraternal twins?