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Chi Square Test

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Chi Square Test

Gitanjali Batmanabane

- Prepare a contingency table
- Realise which study designs are suitable for applying the chi square test
- Understand the assumptions / limitations of the chi square test.

Know thyself

Why does he keep saying this all the time?

No, my son, but I understand something about this “NOT KNOWING”

Excuse me sir, you say “know yourself” all the time but do

YOU KNOW YOURSELF?

- Test of proportions
- Non parametric test
- Dichotomous variables are used
- Tests the association between two factors
e.g. treatment and disease

gender and mortality

Relationship

between variables

Not statistically

associated

Statistically

associated

Non-causal

Causal

Indirectly

causal

Directly

causal

Contingency (2X2) table

- Enter number of subjects – not percentages, ratios, averages etc.,
- Each subject can be entered only once

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

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

25

30

25

30

55

Total

- Data is from a random sample.
- A sufficiently large sample size is required (at least 20)
- Actual count data (not percentages)
- Adequate cell sizes should be present. (>5 in all cells- if less number present apply Yates correction)
- Observations must be independent.
- Does not prove causality.