Teaching · For Teachers · Assessment

Do your items
measure one
thing together?

Paste the item scores from a quiz, a rubric or a survey scale. The calculator gives Cronbach’s alpha with the working, then goes item by item to show which questions pull with the rest of the test and which ones do not.

Calculate alphaWhat you get
$12one-time · every device you sign in on
Buy · $12

Included with Caveat Classroom and Whole-school licenses. Or get it in the For Teachers toolkit, $79 for 72 items.

The alpha calculator

The worked example is made up: ten students on a six-item lab safety quiz scored 0 to 4. Use rows without names. Nothing you type leaves this browser.

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What you get

Teaching · For Teachers$12 · included with Classroom and Whole-school licenses

What you finish with
Cronbach’s alpha (KR-20 for right/wrong items), the mean inter-item correlation, and each item’s mean, variance, item-total correlation and alpha if dropped, with weak items flagged. It prints as a summary and downloads as a spreadsheet.
Who it is for
Teachers and departments writing common assessments, assessment coordinators, teachers running surveys for action research, and graduate students in education.
Format
Browser calculator for a phone, tablet or computer, by touch or keyboard. Up to 100 items and 1,000 students. Prints a summary; downloads a spreadsheet (.csv); export and import a file.
Access
$12 one-time, yours on every device you sign in on; the tool opens once you are signed in with it. Included with Caveat Classroom and Whole-school licenses. Also in the For Teachers toolkit ($79) and the Teaching Suite pass ($199), and 1 more bundle. Everything you type stays in this browser; nothing is sent to Caveat.
Where it lives
The For Teachers space in Caveat Teaching.
Support
Questions, a correction or a request: team@caveatacademic.com.

How to use it

  1. Name the test or scale.
  2. Paste the scores, a row per student, items in the same order.
  3. Read alpha and the steps.
  4. Check flagged items for wording or key problems.
  5. Print the summary or download the spreadsheet.

Calculate alpha

How alpha is worked out

Item spread against total spread.

The worked example, in four numbersItem variances, added8.156spread of each questionTotal score variance36.489spread of the whole testShared share0.7761 − 8.156 ÷ 36.489Alpha0.93× 6 ÷ 5The worked example, in four numbersItem variances, added8.156spread of each questionTotal score variance36.489spread of the whole testShared share0.7761 − 8.156 ÷ 36.489Alpha0.93× 6 ÷ 5
When items rise and fall together, the total varies far more than the items do separately; alpha measures how much more.

The formula

Alpha compares the spread of each item with the spread of the total score. If items measure the same thing, students who do well on one tend to do well on others, so the totals spread out far more than the items alone would suggest. With k items, α = k ÷ (k − 1) × (1 − sum of item variances ÷ variance of totals). For items scored right or wrong, the same formula is KR-20.

Reading it carefully

Alpha belongs to a set of scores from a group, not to a test: the same quiz given to a narrower group gives a lower alpha. It rises with more items, so a long test can reach .90 while measuring a mix of things. Item-total correlations and alpha if dropped point to individual questions worth a second look; they do not prove a question is wrong.

Sources

  1. Cronbach, L. J. (1951). Coefficient alpha and the internal structure of tests. Psychometrika, 16(3), 297–334.
  2. Kuder, G. F., and Richardson, M. W. (1937). The theory of the estimation of test reliability. Psychometrika, 2(3), 151–160.
  3. American Educational Research Association, American Psychological Association, and National Council on Measurement in Education. (2014). Standards for Educational and Psychological Testing, chapter 2.
  4. Tavakol, M., and Dennick, R. (2011). Making sense of Cronbach’s alpha. International Journal of Medical Education, 2, 53–55.

The worked example is made up. Paste scores without names. Your scores stay in this browser. Support: team@caveatacademic.com.