Question 2: Is there any evidence that previous vaginal delivery is related to the risk of a caesarean?
Previous vaginal delivery is a qualitative, dichotomous variable so we can do a chi-squared test or Fisher's exact test.
We get the following output:
| Previous vaginal delivery * Caesarian section Crosstabulation | ||||
|---|---|---|---|---|
| Count | ||||
| Caesarian section | Total | |||
| No | Yes | |||
| Previous vaginal delivery | No | 427 | 122 | 549 |
| Yes | 274 | 14 | 288 | |
| Total | 701 | 136 | 837 | |
| Chi-Square Tests | |||||
|---|---|---|---|---|---|
| Value | df | Asymp. Sig. (2-sided) | Exact Sig. (2-sided) | Exact Sig. (1-sided) | |
| Pearson Chi-Square | 41.840a | 1 | .000 | ||
| Continuity Correctionb | 40.574 | 1 | .000 | ||
| Likelihood Ratio | 49.272 | 1 | .000 | ||
| Fisher's Exact Test | .000 | .000 | |||
| Linear-by-Linear Association | 41.790 | 1 | .000 | ||
| N of Valid Cases | 837 | ||||
| a. 0 cells (.0%) have expected count less than 5. The minimum expected count is 46.80. | |||||
| 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 = 41.84, d.f. = 1, P < 0.001). Hence there is very strong evidence that caesarean section is related to previous vaginal delivery.
We can also find the odds ratio by choosing Risk from Statistics in Crosstabs. We get:
| Risk Estimate | |||
|---|---|---|---|
| Value | 95% Confidence Interval | ||
| Lower | Upper | ||
| Odds Ratio for Previous vaginal delivery (No / Yes) | .179 | .101 | .317 |
| For cohort Caesarian section = No | .818 | .776 | .861 |
| For cohort Caesarian section = Yes | 4.571 | 2.679 | 7.800 |
| N of Valid Cases | 837 | ||
Hence the odds ratio is 0.18 with 95% confidence interval 0.10 to 0.32.
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This page maintained by Martin Bland.
Last updated: 21 February, 2012.