Gitanjali 1

Chi Square TestChi Square Test

Gitanjali BatmanabaneGitanjali Batmanabane

Gitanjali 2

At the end of this session you will be At the end of this session you will be able to:able to:

Prepare a contingency tablePrepare a contingency table

Realise which study designs are suitable Realise which study designs are suitable

for applying the chi square testfor applying the chi square test

Understand the assumptions / limitations Understand the assumptions / limitations

of the chi square test.of the chi square test.

Gitanjali 3

Know thysel

fWhy does he keep

saying this all the time?

Gitanjali 4

Excuse me sir, you say “know

yourself” all the time but do YOU KNOW YOURSELF?

No, my son, but I

understand something about this

“NOT KNOWING”

Gitanjali 5

What is it?What is it?What is it?What is it?

Test of proportionsTest of proportions

Non parametric testNon parametric test

Dichotomous variables are usedDichotomous variables are used

Tests the association between two Tests the association between two factorsfactors

e.g. treatment and diseasee.g. treatment and disease

gender and mortalitygender and mortality

Gitanjali 6

Associations and Causal AssociationsAssociations and Causal AssociationsRelationship

between variablesRelationship

between variables

Not statistically associated

Not statistically associated

Non-causalNon-causal CausalCausal

Indirectly causal

Indirectly causal

Statistically associated

Statistically associated

Directly causal

Directly causal

Gitanjali 7

ExposureOutcome

Yes No

Yes

No

Contingency (2X2) table

Enter number of subjects – not percentages, ratios, averages etc.,

Each subject can be entered only once

Gitanjali 8

Exposure(estrogen)

Outcome (cancer)

Yes No

Yes

No

Out of 25 women who had uterine cancer, 20 claimed to have used estrogens. Out of 30 women without uterine cancer 5 claimed to have used estrogens.

Total

Total

Gitanjali 9

ExposureOutcome

Yes No

Yes 2020 55

No 55 2525

Out of 25 women who had uterine cancer, 20 claimed to have used estrogens. Out of 30 women without uterine cancer 5 claimed to have used estrogens.

Total

Total

25 30

30

25

55

Gitanjali 10

Assumptions / LimitationsAssumptions / Limitations

Data is from a Data is from a random sample.random sample.

A sufficiently A sufficiently large sample sizelarge sample size is required (at is required (at least 20) least 20)

Actual countActual count data (not percentages) data (not percentages)

Adequate cell sizesAdequate cell sizes should be present. (>5 in should be present. (>5 in all cells- if less number present apply Yates all cells- if less number present apply Yates correction)correction)

Observations must be Observations must be independent.independent.

Does Does not prove causality.not prove causality.