How to Calculate Cohen’s d in R

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Cohen’s d is a widely used effect size measure in the field of statistics and psychology. It provides a standardized measure of the difference between two groups or samples. In this comprehensive guide, we will focus on how to calculate Cohen’s d in R, delve into the theoretical underpinnings of this metric, and discuss its practical applications and considerations.


What is Cohen’s d?

Cohen’s d is a measure of effect size used to indicate the standardized difference between two means. It is typically used in psychological and educational research to gauge the size of an effect or a difference independent of sample size.

Why Use Cohen’s d?

Cohen’s d is an essential measure because it provides context for interpreting the difference between two groups. It allows for comparison across different studies or datasets by providing a standardized measure of effect.

Calculating Cohen’s d in R

While R doesn’t have a built-in function for calculating Cohen’s d, it can easily be computed using basic statistical functions. There are also third-party packages such as effsize that provide functions for this purpose.

Using Basic Statistical Functions

Here’s how you can compute Cohen’s d using basic statistical functions in R:

# Sample data
group1 <- c(1, 2, 3, 4, 5)
group2 <- c(3, 4, 5, 6, 7)

# Calculate the means
mean1 <- mean(group1)
mean2 <- mean(group2)

# Calculate the standard deviations
sd1 <- sd(group1)
sd2 <- sd(group2)

# Calculate the sample sizes
n1 <- length(group1)
n2 <- length(group2)

# Calculate the pooled standard deviation
sdpooled <- sqrt(((n1 - 1) * sd1^2 + (n2 - 1) * sd2^2) / (n1 + n2 - 2))

# Calculate Cohen's d
d <- (mean1 - mean2) / sdpooled

Using the cohen.d() Function from the effsize Package

The effsize package in R provides the cohen.d() function, which simplifies the calculation of Cohen’s d.

# Install the package if not already installed

# Load the package

# Sample data
group1 <- c(1, 2, 3, 4, 5)
group2 <- c(3, 4, 5, 6, 7)

# Calculate Cohen's d
d <- cohen.d(group1, group2)

The cohen.d() function returns an object with several values including Cohen’s d, variance, and confidence intervals.

Interpretation of Cohen’s d

Cohen’s d is usually interpreted as follows:

  • Small effect size: d = 0.2
  • Medium effect size: d = 0.5
  • Large effect size: d = 0.8

However, it’s essential to remember that the context matters, and the above values are just general guidelines.

Practical Applications and Considerations

Cohen’s d is widely used in various fields such as psychology, education, medicine, and social sciences.

  • Psychology: To measure the effectiveness of different therapies.
  • Education: To compare the efficacy of different teaching methods.
  • Medicine: To compare the effect of different treatments.

However, Cohen’s d has some limitations and considerations:

  • It does not account for the confidence of the effect size.
  • It is sensitive to sample size.


Cohen’s d is an important metric for measuring the size of an effect or difference between two groups in a standardized way. R provides both custom and packaged solutions for calculating Cohen’s d. It’s essential to interpret Cohen’s d with consideration of the context and the nature of the data. Being mindful of its limitations, it’s a valuable tool in the arsenal of any data analyst or researcher.

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