ANCOVA
Suppose a researcher randomly assigns 30 police officers to receive one of two different “dosages” of cognitive interview intervention to enhance eyewitness recollections or to be assigned into a control “business as usual” group. She determines the number of details that each police officer can obtain from 5 eyewitnesses of armed robbery. Further, these details are weighted such that important details are given 10 points, minor but correct details are given 5 points, and incorrect or irrelevant details are given 0 points. Also, she obtained the percentage of cases that were classified as solved and closed by each of the police officers during the last 12 months before the study began. This covariate was included to determine if any group differences in details of a crime by eyewitnesses persisted when police officer’s estimated eyewitness interview skills were statistically controlled for. Her results are as follows:
One Cognitive Interview Session
# of Details % Solved Two Cognitive Interview Sessions
# of details % Solved “Business as Usual”
# of Details % Solved
10.00 100.00 56.00 118.00 24.00 93.00
28.00 105.00 39.00 110.00 38.00 79.00
37.00 111.00 61.00 105.00 23.00 82.00
31.00 94.00 28.00 120.00 29.00 92.00
15.00 95.00 28.00 111.00 32.00 78.00
15.00 76.00 23.00 96.00 19.00 89.00
12.00 88.00 46.00 126.00 24.00 89.00
25.00 77.00 46.00 96.00 33.00 109.00
34.00 99.00 50.00 133.00 36.00 100.00
33.00 99.00 47.00 130.00 27.00 101.00
Please enter the data above into SPSS to answer some of these questions below:
a. What are the categorical independent variable? (1 oint)
b. What is the dependent variable? (1 point)
c. What is the covariate? (1 point)
d. What are the null and alternative hypotheses concerning the potential effects of the treatment conditions without the covariate? (1 point)
e. What are the null and alternative hypotheses concerning the potential effects of the treatment conditions with the covariate included in the analysis? (1 point)
f. Which two groups would you compare to determine if dosage was an important consideration for use of the Cognitive Interview in the field? (1 point)
g. Run an analysis of variance with the covariate. Include the eta squared (η2) in the output, label each of the between participants conditions for credit (paste the ANOVA table results from SPSS in your hw for credit). (1 point)
h. What is your decision regarding the null hypothesis concerning the effects of the interventions (i.e., did you reject or fail to reject the null)? Explain your decision. (1 point)
i. Was this study sufficiently powered? In other words, were there enough participants in this study to warrant strong conclusions or might the sample size be a potential limitation? (show the G*Power output as part of your answer to receive credit). 1 point
