Examples of Published Research With Analysis Including a Two-Sample
Comparison of Means Using the t Statistic
Candice Lewis provided an example of a two sample I test: Ford D. Y., and Harris,
J.J. (1997). A study of the racial identity and achievement of black males and females,
Roeper Review, 20, 105-110.
There were 149 participants (54 male participants and 95 female participants).
The participants were black students in grades 6 through grade 9. One of the questions
this study asked was, “How do black males and females differ in their racial identities?”
The variables studied were gender and racial identity. Ford and Harris (1997) used the
revised Racial Identity Scale for Black Students (RIS) to assess students’ racial identity.
The RIS was administered to all of the students. The revised scale contained 24 Likerttype questions (strongly agree=4 to strongly disagree=1). The RIS is divided into four
subscales: pre-encounter, encounter, immersion-emersion, and internalization.
The pre-encounter subscale contained 5 items such as: I am ashamed to be an
African American, sometimes I feel like other students do not like me because I am
African American. The encounter subscale contained eight items such as: I try to learn
more about African Americans by talking to other people about my heritage, I am
determined to find my Black identity. The immersion-emersion subscale contained eight
items such as: Black is beautiful, My future is tied to the future of other Blacks. The
internalization subscale contained three items such as everybody should learn about
the cultures of other groups.
A mean of 2.0 was found for the pre-encounter subscale which means the
students did not agree with these items. According to the t test, both male participants
and female participants had a mean of 2.0 on this sub-scale, therefore the results were
not significantly different. A mean of 2.8 was found on the encounter sub-scale. Male
participants and female participants had the same mean (2.8), so the results were not
statistically significant and students tended to agree with items in the encounter subscale. Results of the immersion-emersion t test revealed significant gender difference (p
< .05), with male participants having a higher response of 3.3. Female participants had
a mean response of 3.1. The highest racial identity mean was 3.6 for the internalization
sub-scale. According to the t test results, females had a significantly higher mean
response of 3.7, compared to males, whose mean was 3.5, (p <. 05).
As far as I can tell this is an appropriate example of an independent samples t
test. I am assuming they used the appropriate t test for unequal sample sizes.
Thank you, Candice. Here is my critique:
1. The authors did not report the df.
2. The authors used dehumanizing language ("males" and "females").
3. The authors did not report exact p-values.
T-Examples.docx
4. The authors did not report the standard deviations. Without the standard deviations
we cannot determine whether the significant effect reported here is trivial in magnitude
or not. I suspect it is trivial. We also cannot determine whether the appropriate t test
was employed (was there homogeneity of variance?).
5. There certainly appears to be a lot of crap out there in the journals you all read.
6. I would have liked to see a quote of the author's summary statements.
Here is another summary provided by a previous student:
Chng, C., Carlon, A., & Toynes, B. (2006). HIV on historically black colleges and
universities (HBCU): A study of five campuses in Texas, Oklahoma,
Louisiana. College Student Journal, 40(1), 25-34. Retrieved from PsycINFO
database.
Subjects: The subjects were students at a private Historically Black College or
University
Variables Studied: (1) Gender of Student (2) HIV knowledge.
Design: Empirical Observational study. The researcher surveyed 1,146 students on
their knowledge of HIV.
Conclusions: Female students know more about HIV than Male students
Quote: “Female students (Mean = 6.47. S.D. = 1.06) scored higher on overall HIV
knowledge than did their male counterparts (Mean = 6.11, S.D. = 1.16).”… ”As
seen in Table 2, we found statistical significance for gender on the overall HIV
knowledge scores where women scored higher than men (Men = 6.19, Women =
6.47, t(519) = -2.887, p̲ = .004).”
The standard deviations appear to be approximately 1.5. With a difference between
means of .36 that gives an estimated d of about .24, a small difference.