Cronbach's Alpha Calculator
Compute Cronbach's alpha to measure the internal consistency of a survey, test, or scale. Choose between the correlation-based formula or the variance-based formula.
What is Cronbach's Alpha?
Cronbach's alpha measures how closely related a set of items are as a group. It is the most widely used reliability statistic in social science, psychology, and education research.
Formulas:
Variance-based:
α = (k / (k-1)) x (1 - Σσ²ᵢ / σ²total)
Standardized (correlation-based):
α = k x r / (1 + (k-1) x r)
Where k = number of items, σ²ᵢ = variance of item i, σ²total = total test variance, and r = average inter-item correlation.
Interpreting Alpha:
| Alpha Range | Internal Consistency |
|---|---|
| 0.90+ | Excellent |
| 0.80 - 0.89 | Good |
| 0.70 - 0.79 | Acceptable |
| 0.60 - 0.69 | Questionable |
| 0.50 - 0.59 | Poor |
| Below 0.50 | Unacceptable |
Important Notes:
- Alpha increases with more items, so a high alpha does not always mean the items measure a single construct
- Alpha can be negative if item variances exceed total variance, which suggests reverse-coded items or data problems
- For scales shorter than 10 items, alpha tends to underestimate true reliability