Exercise: Caesarean section and obesity, 3

Question 3: Is there any evidence that induction of labour is related to the risk of a caesarean?

Suggested answer

Induction of labour is a qualitative, dichotomous variable so we can do a chi-squared test or Fisher's exact test.

We get the following output:

Induction of labour * Caesarian section Crosstabulation
  Caesarian section Total
No Yes
of labour
No 546 92 638
Yes 155 46 201
Total 701 138 839

Chi-Square Tests
  Value df Asymp. Sig.
Exact Sig.
Exact Sig.
Pearson Chi-Square 7.971 a 1 .005
Continuity Correctionb 7.366   1 .007
Likelihood Ratio 7.509   1 .006
Fisher's Exact Test       .006 .004
Linear-by-Linear Association 7.961   1 .005
N of Valid Cases 839  
a. 0 cells (.0%) have expected count less than 5. The minimum expected count is 33.06..
b. Computed only for a 2x2 table

The chi-squared test is valid, because the expected frequencies are all greater than five. The relationship is highly significant (chi-squared = 7.97, d.f. = 1, P = 0.005). Hence there is strong evidence that caesarean section is related to induction of labour.

We can also find the odds ratio by choosing Risk from Statistics in Crosstabs. We get:

Risk Estimate
    Value   95% Confidence
Lower Upper
Odds Ratio for Induction
of labour (No / Yes)
1.761 1.185 2.618
For cohort Caesarian
section = No
1.110 1.023 1.204
For cohort Caesarian
section = Yes
.630 .459 .865
N of Valid Cases 837

Hence the odds ratio is 1.76 with 95% confidence interval 1.19 to 2.62.

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Last updated: 21 February, 2012.

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