DIS LECTURE 3 NOTES

Reliability Analysis: Understanding Cronbach's Alpha

Introduction

• Instituted By: Vrije Universiteit Amsterdam, Faculty of Social Sciences

• Objective: Analyze the reliability of a measurement tool using Cronbach's Alpha.

What is a Good Alpha?

• Dependence on Context: Definition of a good alpha varies among experts.

• Rule of Thumb: Commonly, a good alpha is considered to be greater than 0.8.

• Item Correlation and Number of Items:

  • Alpha is influenced by inter-item correlation.

  • More items generally lead to a higher alpha.

  • Thus, a value of 0.8 may be acceptable under certain conditions depending on these aspects.

Increasing Reliability

Criteria for Improvement

1. Cronbach’s Alpha if Item Deleted

   • Evaluate the alpha's value if a particular item is removed.

   • Use this comparison to establish whether removing helps in increasing reliability.

2. Corrected Item-Total Correlation

   • This measures the relationship between an individual item and the total score from other items.

   • Acceptable Standard: Should be greater than 0.5.

3. Squared Multiple Correlation

   • Computes the proportion of variance in an item explained by other items in the scale.

   • Acceptable Standard: Should be greater than 0.3.

Detailed Breakdown of Criteria

1. Cronbach’s Alpha if Item Deleted

• Compare this value with the overall Cronbach’s Alpha (e.g., 0.866).

2. Corrected Item-Total Correlation

• Assess how much an item correlates with the total score from the other items.

• A value greater than 0.5 indicates strong correlation.

3. Squared Multiple Correlation (R²)

• Represents the R² from a regression analysis where this item is treated as a dependent variable, with other items as predictors.

• A threshold of 0.3 is deemed acceptable.

Decision Making for Item Removal

Guidelines for Removal

• If an item fails to meet the criteria:

  • A low Corrected Item-Total Correlation (< 0.5) and/or

  • A low Squared Multiple Correlation (< 0.3)

• Additionally, assess the impact on overall Cronbach's Alpha when considered for removal.

• Importance of Theoretical Meaning: Consider if item removal makes sense based on the concept being measured.

Example Items for Assessment

• Questions on emotional responses:

  1. Losing patience

  2. Being irritated by small things

  3. Remaining angry

  4. Feeling like shouting or taking revenge

  5. Perceptions of unfairness or jealousy

• Assess the distribution of responses for interpreting reliability and validity.

Final Notes

Interpretation Importance

• Always contextualize and interpret statistical results meaningfully within the broader framework of the research objectives.