Running a T-test
Gabi Hanna
2025-06-03
1 Loading Libraries
# install any packages you have not previously used, then comment them back out.
#install.packages("car")
#install.packages("effsize")
library(psych) # for the describe() command
library(car) # for the leveneTest() command## Loading required package: carData##
## Attaching package: 'car'## The following object is masked from 'package:psych':
##
## logitlibrary(effsize) # for the cohen.d() command##
## Attaching package: 'effsize'## The following object is masked from 'package:psych':
##
## cohen.d2 Importing Data
d <- read.csv(file="Data/projectdata.csv", header=T)
# For the HW, you will import the project dataset you cleaned previously
# This will be the dataset you'll use for HWs throughout the rest of the semester3 State Your Hypothesis
We predict that there will be a significant difference in general self-efficacy by level of education between having a high school diploma or less and having completed some graduate degree.
4 Check Your Variables
# you **only** need to check the variables you're using in the current analysis
## Checking the Categorical variable (IV)
str(d)## 'data.frame': 2163 obs. of 7 variables:
## $ ResponseID: chr "R_BJN3bQqi1zUMid3" "R_2TGbiBXmAtxywsD" "R_12G7bIqN2wB2N65" "R_39pldNoon8CePfP" ...
## $ age : chr "1 between 18 and 25" "1 between 18 and 25" "1 between 18 and 25" "1 between 18 and 25" ...
## $ edu : chr "2 Currently in college" "5 Completed Bachelors Degree" "2 Currently in college" "2 Currently in college" ...
## $ swb : num 4.33 4.17 1.83 5.17 3.67 ...
## $ support : num 6 6.75 5.17 5.58 6 ...
## $ efficacy : num 3.4 3.4 2.2 2.8 3 2.4 2.3 3 3 3.7 ...
## $ stress : num 3.3 3.3 4 3.2 3.1 3.5 3.3 2.4 2.9 2.7 ...# if the categorical variable you're using is showing as a "chr" (character), you must change it to be a ** factor ** -- using the next line of code (as.factor)
d$edu <- as.factor(d$edu)
str(d)## 'data.frame': 2163 obs. of 7 variables:
## $ ResponseID: chr "R_BJN3bQqi1zUMid3" "R_2TGbiBXmAtxywsD" "R_12G7bIqN2wB2N65" "R_39pldNoon8CePfP" ...
## $ age : chr "1 between 18 and 25" "1 between 18 and 25" "1 between 18 and 25" "1 between 18 and 25" ...
## $ edu : Factor w/ 7 levels "1 High school diploma or less, and NO COLLEGE",..: 2 5 2 2 2 2 5 2 2 2 ...
## $ swb : num 4.33 4.17 1.83 5.17 3.67 ...
## $ support : num 6 6.75 5.17 5.58 6 ...
## $ efficacy : num 3.4 3.4 2.2 2.8 3 2.4 2.3 3 3 3.7 ...
## $ stress : num 3.3 3.3 4 3.2 3.1 3.5 3.3 2.4 2.9 2.7 ...table(d$edu, useNA = "always")##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 2 Currently in college
## 1783
## 3 Completed some college, but no longer in college
## 24
## 4 Complete 2 year College degree
## 133
## 5 Completed Bachelors Degree
## 75
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0## Checking the Continuous variable (DV)
# you can use the describe() command on an entire dataframe (d) or just on a single variable within your dataframe -- which we will do here
describe(d$efficacy)## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 2163 3.11 0.44 3.1 3.12 0.44 1.2 4 2.8 -0.19 0.36 0.01# also use a histogram to visualize your continuous variable
hist(d$efficacy)
# use the describeBy() command to view the means and standard deviations by group
# it's very similar to the describe() command but splits the dataframe according to the 'group' variable
describeBy(d$efficacy, group=d$edu)##
## Descriptive statistics by group
## group: 1 High school diploma or less, and NO COLLEGE
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 38 3.19 0.47 3.05 3.21 0.52 2.1 4 1.9 -0.18 -0.69 0.08
## ------------------------------------------------------------
## group: 2 Currently in college
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 1783 3.09 0.44 3 3.09 0.44 1.2 4 2.8 -0.21 0.53 0.01
## ------------------------------------------------------------
## group: 3 Completed some college, but no longer in college
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 24 3.28 0.48 3.35 3.3 0.52 2.2 4 1.8 -0.3 -0.83 0.1
## ------------------------------------------------------------
## group: 4 Complete 2 year College degree
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 133 3.21 0.43 3.2 3.22 0.44 2 4 2 -0.2 -0.51 0.04
## ------------------------------------------------------------
## group: 5 Completed Bachelors Degree
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 75 3.25 0.38 3.2 3.23 0.3 2.3 4 1.7 0.25 -0.38 0.04
## ------------------------------------------------------------
## group: 6 Currently in graduate education
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 78 3.14 0.47 3.15 3.15 0.52 2 4 2 -0.07 -0.58 0.05
## ------------------------------------------------------------
## group: 7 Completed some graduate degree
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 32 3.32 0.53 3.2 3.35 0.67 2.1 4 1.9 -0.2 -1.07 0.09# lastly, use a boxplot to examine your chosen continuous and categorical variables together
boxplot(d$efficacy~d$edu)
5 Check Your Assumptions
5.1 T-test Assumptions
- IV must have two levels
- Data values must be independent (independent t-test only)
- Data obtained via a random sample
- Dependent variable must be normally distributed
- Variances of the two groups are approx. equal
# If the IV has more than 2 levels, you must DROP any additional levels in order to meet the first assumption of a t-test.
## NOTE: This is a FOUR STEP process!
d <- subset(d, edu != "2 Currently in college") # use subset() to remove all participants from the additional level
table(d$edu, useNA = "always") # verify that now there are ZERO participants in the additional level##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 2 Currently in college
## 0
## 3 Completed some college, but no longer in college
## 24
## 4 Complete 2 year College degree
## 133
## 5 Completed Bachelors Degree
## 75
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d$edu <- droplevels(d$edu) # use droplevels() to drop the empty factor
table(d$edu, useNA = "always") # verify that now the entire factor level is removed ##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 3 Completed some college, but no longer in college
## 24
## 4 Complete 2 year College degree
## 133
## 5 Completed Bachelors Degree
## 75
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d <- subset(d, edu != "3 Completed some college, but no longer in college") # use subset() to remove all participants from the additional level
table(d$edu, useNA = "always") # verify that now there are ZERO participants in the additional level##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 3 Completed some college, but no longer in college
## 0
## 4 Complete 2 year College degree
## 133
## 5 Completed Bachelors Degree
## 75
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d$edu <- droplevels(d$edu) # use droplevels() to drop the empty factor
table(d$edu, useNA = "always") # verify that now the entire factor level is removed ##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 4 Complete 2 year College degree
## 133
## 5 Completed Bachelors Degree
## 75
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d <- subset(d, edu != "4 Complete 2 year College degree") # use subset() to remove all participants from the additional level
table(d$edu, useNA = "always") # verify that now there are ZERO participants in the additional level##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 4 Complete 2 year College degree
## 0
## 5 Completed Bachelors Degree
## 75
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d$edu <- droplevels(d$edu) # use droplevels() to drop the empty factor
table(d$edu, useNA = "always") # verify that now the entire factor level is removed ##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 5 Completed Bachelors Degree
## 75
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d <- subset(d, edu != "5 Completed Bachelors Degree") # use subset() to remove all participants from the additional level
table(d$edu, useNA = "always") # verify that now there are ZERO participants in the additional level##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 5 Completed Bachelors Degree
## 0
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d$edu <- droplevels(d$edu) # use droplevels() to drop the empty factor
table(d$edu, useNA = "always") # verify that now the entire factor level is removed ##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 6 Currently in graduate education
## 78
## 7 Completed some graduate degree
## 32
## <NA>
## 0d <- subset(d, edu != "6 Currently in graduate education") # use subset() to remove all participants from the additional level
table(d$edu, useNA = "always") # verify that now there are ZERO participants in the additional level##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 6 Currently in graduate education
## 0
## 7 Completed some graduate degree
## 32
## <NA>
## 0d$edu <- droplevels(d$edu) # use droplevels() to drop the empty factor
table(d$edu, useNA = "always") # verify that now the entire factor level is removed ##
## 1 High school diploma or less, and NO COLLEGE
## 38
## 7 Completed some graduate degree
## 32
## <NA>
## 0## Repeat ALL THE STEPS ABOVE if your IV has more levels that need to be DROPPED. Copy the 4 lines of code, and replace the level name in the subset() command.5.2 Testing Homogeneity of Variance with Levene’s Test
We can test whether the variances of our two groups are equal using Levene’s test. The NULL hypothesis is that the variance between the two groups is equal, which is the result we WANT. So when running Levene’s test we’re hoping for a NON-SIGNIFICANT result!
# use the leveneTest() command from the car package to test homogeneity of variance
# it uses the same 'formula' setup that we'll use for our t-test: formula is y~x, where y is our DV and x is our IV
leveneTest(efficacy~edu, data = d)## Levene's Test for Homogeneity of Variance (center = median)
## Df F value Pr(>F)
## group 1 1.1278 0.292
## 68Levene’s test revealed that our data does not have significantly different variances between the two comparison groups, having a high school diploma or less and no college and having completed some graduate degree, on their levels of self-efficacy.
When running a t-test, we can account for heterogeneity in our variance by using the Welch’s t-test, which does not have the same assumption about variance as the Student’s t-test (the general default type of t-test in statistics). R defaults to using Welch’s t-test so this doesn’t require any changes on our part! Even if your data has no issues with homogeneity of variance, you’ll still use Welch’s t-test – it handles the potential issues around variance well and there are no real downsides. We’re using Levene’s test here to get into the habit of checking the homogeneity of our variance, even if we already have the solution for any potential problems.
5.3 Issues with My Data
My independent variable has more than two levels. To proceed with this analysis, I will drop the participants from my sample who are currently in college, have completed some college but are no longer in college, have completed a 2 year college degree, have completed a Bachelors degree, and those who are currently in graduate education. I will make a note to discuss this issue in my Methods section write-up and in my Discussion section as a limitation of my study.
My data has no issue regarding homogeneity of variance, as Levene’s test was not significant. I will use Welch’s t-test instead of Student’s t-test in my analysis.
6 Run a T-test
# Very simple! we use the same formula of y~x, where y is our DV and x is our IV
t_output <- t.test(d$efficacy~d$edu) # t_output will now show in your Global Environment7 View Test Output
t_output##
## Welch Two Sample t-test
##
## data: d$efficacy by d$edu
## t = -1.0248, df = 62.69, p-value = 0.3094
## alternative hypothesis: true difference in means between group 1 High school diploma or less, and NO COLLEGE and group 7 Completed some graduate degree is not equal to 0
## 95 percent confidence interval:
## -0.3658669 0.1178406
## sample estimates:
## mean in group 1 High school diploma or less, and NO COLLEGE
## 3.194737
## mean in group 7 Completed some graduate degree
## 3.3187508 Calculate Cohen’s d - Effect Size
# once again, we use the same formula, y~x, to calculate cohen's d
# We **only** calculate effect size if the test is SIG!
# d_output <- cohen.d(d$efficacy~d$edu) # d_output will now show in your Global Environment9 View Effect Size
# d_output
## Remember to always take the ABSOLUTE VALUE of the effect size value (i.e., it will never be negative)10 Write Up Results
To test our hypothesis that there would be a significant difference in general self-efficacy by level of education between having a high school diploma or less and no college, and having completed some graduate degree, we used an independent samples t-test. This required us to drop our participants who are currently in college, have completed some college but are no longer in college, have completed a 2 year college degree, have completed a Bachelors degree, and those who are currently in graduate education from our sample, as we are limited to a two-group comparison when using this test. We tested the homogeneity of variance with Levene’s test and found signs of homogeneity (p > 0.05). We used Welch’s t-test and our data met all assumptions of an independent samples t-test.
Contrary to our predictions, we found that those with a high school diploma or less and no college (M = 3.19, SD = 0.47) did not have significantly different levels of self-efficacy than those who completed some graduate degree (M = 3.32, SD = 0.53); t(62.69) = -1.02, p > 0.05 (see Figure 1). The effect size was not calculated using Cohen’s d, because the t-test was not significant (Cohen, 1988).
References
Cohen J. (1988). Statistical Power Analysis for the Behavioral Sciences. New York, NY: Routledge Academic.