Cronbach's Alpha

Definition

Cronbach's alpha is a coefficient of internal consistency reliability that measures how closely the items in a scale or test are related to one another. Values range from 0 to 1, with higher values indicating greater reliability. A commonly accepted minimum threshold is 0.70 for exploratory research and 0.80 for confirmatory or high-stakes applications.

Why It Matters

When you construct a questionnaire or psychometric scale, you assume that all items measure the same underlying construct. Cronbach's alpha provides a single-number summary of whether the items hang together. A low alpha suggests that some items may be measuring something different, that the scale has too few items, or that the response format is confusing. Reporting alpha is a standard requirement in scale-validation studies.

Example

A researcher develops a 10-item scale measuring organisational trust. After piloting the scale with 200 employees, she computes Cronbach's alpha and obtains 0.85. She then examines the "alpha if item deleted" statistics and finds that removing item 7 raises alpha to 0.91. She concludes that item 7 is poorly worded or taps into a different dimension, and she revises it before the main study.

Related Terms

• Reliability
• Internal Consistency
• Construct Validity
• Factor Analysis

Software Notes

• SPSS: Analyze > Scale > Reliability Analysis. Move all scale items to the Items box. Under Statistics, check Scale if item deleted, Item-total statistics, and Descriptives for scale.
• R: psych::alpha(data.frame(item1, item2, ...)) returns Cronbach's alpha, item-total correlations, and alpha-if-deleted values. psych::omega() computes McDonald's omega as a modern alternative.
• Stata: alpha item1 item2 item3, item detail computes Cronbach's alpha and item-total correlations. alpha item1 item2 item3, item asis suppresses automatic reverse-coding.