Free item analysis calculator under Classical Test Theory: paste your response matrix (one row per subject, one column per item) and it returns, per item, difficulty (p or mean), standard deviation, corrected item-total correlation, 27% extreme-group discrimination (Kelley, 1939) and alpha-if-deleted; and for the whole test, Cronbach's alpha (which with 0/1 items is exactly KR-20), mean, SD and the standard error of measurement. It auto-detects dichotomous (exam) versus polytomous (Likert) data, rates each item with the Ebel and Frisbie (1991) thresholds and drafts the APA 7 results sentence.
Worked example: in an 8-question exam taken by 25 students (mean 4.64, SD 1.63), overall reliability was KR-20 = .37. Question 5 looked normal by difficulty (p = .52), but its discrimination was negative (D = −.57: 2 of the 7 best students answered it correctly versus 6 of the 7 weakest) and its corrected item-total correlation was −.70. Removing it raises KR-20 from .37 to .69: the answer key was wrong or the stem was ambiguous.
Cronbach's alpha or KR-20? They are the same coefficient when items are scored 0/1: KR-20 is the dichotomous case of alpha. What to do with a negatively discriminating item? First check the answer key and any reverse-worded items left unrecoded; if the key is correct, void it (exam) or drop/rewrite it (scale).
Find which items of your exam or scale drag reliability down: difficulty, extreme-group discrimination, corrected item-total and alpha-if-deleted.
If you use it in your thesis or in a paper, cite it so that readers of your Methods section know how the calculation was done.
Ciudad-Fernández, V. (2026). Item analysis calculator [Web application]. Ciudad Science. Retrieved October 14, 2026, from https://analisisdedatospsicologia.com/en/tools/item-analysis-calculator
For your Methods section: Item difficulty and discrimination indices and test reliability (Kuder & Richardson, 1937; Cronbach, 1951) were computed with the Ciudad Science item analysis calculator (Ciudad-Fernández, 2026).
@misc{ciudadfernandez2026analisisitems,
author = {Ciudad-Fern{\'a}ndez, V{\'i}ctor},
title = {{Item analysis calculator}},
year = {2026},
publisher = {Ciudad Science},
howpublished = {Web application},
url = {https://analisisdedatospsicologia.com/en/tools/item-analysis-calculator},
urldate = {2026-10-14},
note = {Accessed 2026-10-14},
}