How to Calculate Cohen’s Kappa in R

Cohen’s Kappa is a statistical measure used to evaluate the inter-rater reliability or the extent of agreement among raters. It is widely used in various fields such as healthcare, research, and machine learning. In this comprehensive guide, we will focus on how to calculate Cohen’s Kappa in R, understand the theoretical background, and discuss its applications and considerations.

Introduction

What is Cohen’s Kappa?

Cohen’s Kappa (κ) is a statistic that measures the inter-rater reliability for qualitative (categorical) items. It evaluates how well two or more judges/raters can make the same decision or classification. Kappa statistics are used not only to evaluate a single classification but also to assess multiple raters working with categorical schemes.

Kappa takes into account the possibility of the agreement occurring by chance, which is not considered in simple percent agreement. The Kappa statistic is always less than or equal to 1. A Kappa of 1 indicates perfect agreement, whereas a Kappa of 0 indicates agreement equivalent to chance.

Why Use Cohen’s Kappa?

1. Chance Correction: Unlike plain accuracy, Kappa takes into account the possibility of the agreement occurring by chance.
2. Robustness and Reliability: Kappa is relatively robust to sample size compared to plain accuracy.
3. Multi-rater Evaluation: It is capable of evaluating the agreement among more than two raters.

Calculating Cohen’s Kappa in R

R offers various libraries for calculating Cohen’s Kappa. We will be looking at two primary methods: using the psych package, and the irr package.

Using the psych Package

First, you need to install and load the psych package.

# Install the package if not already installed
install.packages("psych")

# Load the package
library(psych)

Now, use the cohen.kappa() function to calculate Cohen’s Kappa.

# Sample data (Each row represents an item, each column is a rater/judge)
data <- matrix(c(1, 1, 2, 2, 1, 2, 2, 1, 2, 2, 1, 1), ncol=2)

# Calculate Cohen's Kappa
kappa_result <- cohen.kappa(data)

# Display the result
print(kappa_result)

Using the irr Package

Similarly, you can use the irr package to calculate Cohen’s Kappa.

# Install the package if not already installed
install.packages("irr")

# Load the package
library(irr)

# Sample data (Each row represents an item, each column is a rater/judge)
data <- matrix(c(1, 1, 2, 2, 1, 2, 2, 1, 2, 2, 1, 1), ncol=2)

# Calculate Cohen's Kappa
kappa_result <- kappam.fleiss(data)

# Display the result
print(kappa_result)

Interpretation of Cohen’s Kappa

Interpreting the value of Kappa requires some judgment. Here is a rough guideline proposed by Landis and Koch (1977):

• Less than 0: Poor agreement
• 0.00 – 0.20: Slight agreement
• 0.21 – 0.40: Fair agreement
• 0.41 – 0.60: Moderate agreement
• 0.61 – 0.80: Substantial agreement
• 0.81 – 1.00: Almost perfect agreement

Practical Applications and Considerations

Cohen’s Kappa is widely used in:

• Healthcare: Evaluating the agreement of diagnoses made by different doctors.
• Machine Learning: In classification tasks for evaluating the reliability of annotators.
• Social Sciences: Evaluating agreement among observers in studies.

However, it’s essential to understand the underlying assumptions and limitations:

• Cohen’s Kappa requires the categories to be mutually exclusive.
• It does not account for partial agreement which might be relevant in some cases.
• Kappa can be affected by the prevalence of each category (high prevalence can result in high agreement by chance).

Conclusion

Cohen’s Kappa is a robust measure used for evaluating the agreement among two or more raters, especially useful since it accounts for agreement occurring by chance. In R, the psych and irr packages provide efficient and straightforward methods for calculating Cohen’s Kappa. However, users should be cautious and attentive to the underlying assumptions and context-specific nuances when interpreting the results.