Compute the chi-square statistic, and assess the statistical significance for the relationship between smoking and time to pregnancy in given Exercise. State a conclusion about the relationship.

Exercise
Researchers asked women who were pregnant with planned pregnancies how long it took them to get pregnant (Baird and Wilcox, 1985; see also Weiden and Gladen, 1986). Length of time to pregnancy was measured according to the number of cycles between stopping birth control and getting pregnant. Women were also categorized on whether or not they smoked, with smoking defined as having at least one cigarette per day for at least the first cycle during which they were trying to get pregnant. The following table summarizes the observed counts.

a. Among smokers, what percentage were pregnant after the first cycle?

b. Among nonsmokers, what percentage were pregnant after the first cycle?

c. Draw a bar graph that can be used to examine the relationship between smoking and how long it took to get pregnant.

d. Among those who were pregnant after the first cycle, what percentage smoked?

e. Among those who took two or more cycles to become pregnant, what percentage smoked?

f. Do you think there is a relationship between smoking and how long it takes to get pregnant? Explain.

g. Can these data be used as evidence of a causal connection between smoking and how long it takes to get pregnant? Explain.