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.