APA Summary for t-test
1
APA Sample Summary for t-test
Anthony R. Napoli
A Quasi-Experimental Non-Equivalent control group design study (Cook & Campbell,
1979) was conducted to assess the effectiveness of a Scholastic Aptitude Test (SAT) review
course. The sample consisted of 40 senior high school students. Half of the students (n=20)
voluntarily registered for and completed the Zeplin SAT review course (SAT-Review Group)
prior to taking the SAT test. Half of the students did not participate in an SAT review course
(Control Group; n = 20) prior to taking the SAT exam. Gender was equally distributed within
and between each group. The mean age of students in the SAT-Review Group equaled 18.7
years (SD = 2.93), the mean age of students in the Control group = 18.3 years (SD = 2.72).
Students in the SAT Review group had a mean High School average of 80.1% (SD = 7.08),
students in the Control group had a mean High School average of 71.5% (SD = 10.2). Both
groups took the SAT test in May, 2005, just prior to graduating from high school.
An exploratory data analysis was conducted to determine if the SAT score distribution
was normally distributed. Results for the Kolmogorov-Smirnov1 test for normality (Field, 2007)
indicated that the SAT distribution did not deviate significantly from a normal distribution (D =
.084, p = .200). For the sample of 40 students the mean SAT score equaled 1037.7 (SD = 306.6).
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The Kolmogorov-Smirnov test for normality was conducted using the SPSS Descriptive Statistics-Explore
procedure suing the following syntax:
EXAMINE
VARIABLES=SATTotal
/PLOT BOXPLOT STEMLEAF NPPLOT
/COMPARE GROUP
/STATISTICS DESCRIPTIVES
/CINTERVAL 95
/MISSING LISTWISE
/NOTOTAL.
APA Summary for t-test
2
The statistical comparison of the SAT scores for the SAT-Review Group and the Control
students was conducted using SPSS software version 14.0 (Field, 2005). The analysis employed
a t-test for independent groups, with alpha set at the 5% level, two tail test; and tested the null
hypothesis that the groups performed statistically equivalent on the SAT test.
Levene’s test of homogeneity of variance was employed and confirmed that the variances
in SAT test scores for the SAT Review group and Control students were statistically equivalent
(F (38) = 64, p = .429). Results for the means comparison test indicated that the mean SAT
score for SAT Review group (M = 1145.4; SD = 274.5) was significantly greater (t (38) = 2.39, p
= .02) than the mean SAT score for the Control students (M = 925.90; SD = 303.7). On average,
students that participated in the SAT review class scored approximately 220 points higher on the
SAT test than their counterparts in the control group. To assess this effect Cohen’s d effect size
statistics was computed. Based on the recommendations of Cohen (Aron & Aron; 1994) the
resulting of effect size of (d=) .76, reflects a large effect for the SAT review course.
In summary, assuming that the both students groups were statistically equivalent in
academic aptitude, intellect , prior academic achievement, and motivation to perform well on the
SAT test, the statistical evidence presented supports the hypothesis that the Zeplin SAT review
course results in significantly, and meaningfully, higher SAT performance.
References
Aron, A. Aron E. N. (1994) Statistics for Psychology. Englewood Cliffs, NJ: Prentice Hall
Publishers.
Cook, T.D., and Campbell, D.T. 1979. Quasi-Experimentation: Design & Analysis Issues
for Field Settings. New York: Houghton Mifflin.
Field, A. (2005).Discovering Statistics Using SPSS (2nd edition). London: Sage.