Running a T-test
Jocelyn Weddle
2026-03-29
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 Generalized Anxiety Scores between individuals under 18 and young adults aged 18-25.
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': 675 obs. of 7 variables:
## $ X : int 520 2814 3146 3295 717 6056 4753 5365 2044 1965 ...
## $ age : chr "1 under 18" "1 under 18" "1 under 18" "1 under 18" ...
## $ mhealth: chr "none or NA" "none or NA" "none or NA" "none or NA" ...
## $ pss : num 2.75 2.25 3 2 1.75 2 1 1.25 3 1.25 ...
## $ phq : num 1.56 1.44 1.11 1.33 1.44 ...
## $ gad : num 1.14 1.29 1 1 1.14 ...
## $ edeq12 : num 1.33 1.08 1 1 1.17 ...# 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$age <- as.factor(d$age)
str(d)## 'data.frame': 675 obs. of 7 variables:
## $ X : int 520 2814 3146 3295 717 6056 4753 5365 2044 1965 ...
## $ age : Factor w/ 2 levels "1 under 18","2 between 18 and 25": 1 1 1 1 1 1 1 1 1 1 ...
## $ mhealth: chr "none or NA" "none or NA" "none or NA" "none or NA" ...
## $ pss : num 2.75 2.25 3 2 1.75 2 1 1.25 3 1.25 ...
## $ phq : num 1.56 1.44 1.11 1.33 1.44 ...
## $ gad : num 1.14 1.29 1 1 1.14 ...
## $ edeq12 : num 1.33 1.08 1 1 1.17 ...table(d$age, useNA = "always")##
## 1 under 18 2 between 18 and 25 <NA>
## 621 54 0## Checking the Continuous variable (DV)
# you can use the describe() command on an entire dataframe (d) or just on a single variable within your dataframe -- which we will do here
describe(d$gad)## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1 1 675 2.2 0.93 2 2.14 1.06 1 4 3 0.42 -1.06 0.04# also use a histogram to visualize your continuous variable
hist(d$gad)
# use the describeBy() command to view the means and standard deviations by group
# it's very similar to the describe() command but splits the dataframe according to the 'group' variable
describeBy(d$gad, group=d$age)##
## Descriptive statistics by group
## group: 1 under 18
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1* 1 621 9.28 6.55 8 8.83 7.41 1 22 21 0.44 -1.05 0.26
## ------------------------------------------------------------
## group: 2 between 18 and 25
## vars n mean sd median trimmed mad min max range skew kurtosis se
## X1* 1 54 10.56 5.7 11 10.66 7.41 1 19 18 -0.1 -1.25 0.78# lastly, use a boxplot to examine your chosen continuous and categorical variables together
boxplot(d$gad~d$age)
5 Check Your Assumptions
5.1 T-test Assumptions
- IV must have two levels.
- Data values must be independent (independent t-test only).
- Data obtained via a random sample.
- Dependent variable must be normally distributed.
- Variances of the two groups are approx. equal.
# If the IV has more than 2 levels, you must DROP any additional levels in order to meet the first assumption of a t-test.
unique(d$age)## [1] 1 under 18 2 between 18 and 25
## Levels: 1 under 18 2 between 18 and 25## NOTE: This is a FOUR STEP process!
d <- subset(d, age %in% c("1 under 18", "2 between 18 and 25")) # use subset() to remove all participants from the additional level
table(d$age, useNA = "always") # verify that now there are ZERO participants in the additional level##
## 1 under 18 2 between 18 and 25 <NA>
## 621 54 0d$age <- droplevels(d$age) # use droplevels() to drop the empty factor
table(d$age, useNA = "always") # verify that now the entire factor level is removed ##
## 1 under 18 2 between 18 and 25 <NA>
## 621 54 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(gad~age, data = d)## Levene's Test for Homogeneity of Variance (center = median)
## Df F value Pr(>F)
## group 1 0.3287 0.5666
## 673Levene’s test revealed that our data does not have sig different variances between the two comparison groups, individuals under 18 and young adults between 18-25, on their GAD scores.
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 kept participants under 18 and young adults 18-25. 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 did not show any issues regarding homogeneity of variance, as Levene’s test was significant. Therefore, equal variances was met, using a standard t-test.
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$gad~d$age) # t_output will now show in your Global Environment7 View Test Output
t_output##
## Welch Two Sample t-test
##
## data: d$gad by d$age
## t = -1.8787, df = 63.165, p-value = 0.0649
## alternative hypothesis: true difference in means between group 1 under 18 and group 2 between 18 and 25 is not equal to 0
## 95 percent confidence interval:
## -0.50107015 0.01544788
## sample estimates:
## mean in group 1 under 18 mean in group 2 between 18 and 25
## 2.183115 2.4259268 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$gad~d$age) # d_output will now show in your Global Environment9 View Effect Size
d_output##
## Cohen's d
##
## d estimate: -0.2601909 (small)
## 95 percent confidence interval:
## lower upper
## -0.53911067 0.01872894## Remember to always take the ABSOLAUTE VALUE of the effect size value (i.e., it will never be negative)10 Write Up Results
To test our hypothesis that GAD scores would differ between different age groups, we used an independent-samples t-test. This required us to only keep participants under 18 and young adults 18-25 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 no significant signs (p = .57). This suggests that all equal variances was met. Our data met all other assumptions of an independent samples t-test.
We found that participants under 18 (M = 9.28 , SD = 6.55) reported slightly lower scores than young adults between 18-25 (M = 10.56 , SD = 5.70); t(63.17) = -1.88, p = .065 (see Figure 1). The effect size was calculated using Cohen’s d, with a value of 0.26 (small effect; Cohen, 1988).
References
Cohen J. (1988). Statistical Power Analysis for the Behavioral Sciences. New York, NY: Routledge Academic.