Rates of infant mortality, birth defect, and premature labor are high for babies with low birth weight. There are many factors that may contribute to low birth weight.

In this activity, we use data from a random sample of women who participated in a study in 1986 at the Baystate Medical Center in Springfield, MA. (Source: Hosmer and Lemeshow (2000), Applied Logistic Regression: Second Edition.)

For the 30 women in the study with a history of premature labor, a proportion of 18/30=0.60(60%) had babies with low birth weight. For the remaining 159 women, a proportion of 41/159=0.26(26%) had babies with low birth weight.

We now investigate the following research question: do the data provide evidence that the proportion of babies born with low birth weight is higher for women with a history of premature labor? This question is answered with a hypothesis test. To conduct the test we use a 1% level of significance.

Question 1:

Is this study observational or experimental?

Question 2:

Before analyzing the data, use your own experience and intuition to predict what the data will show. Do you think the proportion of babies with low birth weight is higher for women with a history of premature labor?

Question 3:

We will test the claim that the proportion of women with low birth weight babies is higher among women with a history of premature labor. What are the null and alternative hypotheses?

Question 4:

Are the criteria for approximate normality satisfied?

Instructions

Click on the link corresponding to your statistical package to see instructions for completing the activity, and then answer the questions below.

Excel | TI Calculator

Question 5:

State the test statistic and P-value. Interpret these values.

Question 6:

Give a conclusion in context, and discuss whether a causal conclusion is appropriate.

Use excel 1. Enter labels x1 and x2 in cells A1 and B1.

2. Below these labels, in cells A2 and B2, enter the numbers of successes for the two samples.

3. Enter labels n1 and n2 in cells A3 and B3.

4. Below these labels, in cells A4 and B4, enter the sizes of the two samples.

5. Enter the labels p-hat1 and p-hat2 in cells A5 and B5.

6. In cell A6, enter the formula to compute first sample proportions.

=A2/A4

7. In cell B6, enter the formula to compute first sample proportions.

=B2/B4

8. In cell C1, enter the label p-bar.

9. In cell C2, enter the formula for the pooled proportion.

=(A2+B2)/(A4+B4)

10. In cell C3, enter the label for the test statistic, Z.

11. In cell C4, enter the formula for the test statistic.

=(A6-B6-0)/SQRT(C2*(1-C2)/A4+C2*(1-C2)/B4)

12. In cell C5, enter the label, P-value.

13. In cell C6, enter the formula for the P-value.

=1-NORMSDIST(C4)