A UK study by Peacock et al (1995) of factors affecting the outcome of pregnancy among 1513 women reported that the overall incidence of preterm births was 7.5%, SE 0.68% and 95% CI 6.1 to 8.8%.

(a) What is meant by SE = 0.68%?

(b) What is meant by 95% CI 6.1 to 8.8%?

(c) How would the confi dence interval change if 90% limits were used?

(d) How would the confi dence interval change if 99% limits were used?

Another study conducted at about the same time in Denmark (Kristensen et al, 1995) and including 51 851 women, reported that the overall incidence of pre-term birth was 4.5% (95% CI 4.3 to 4.7%).

(e) Explain why this 95% CI is narrower than that reported in the UK study. Do you think that there is a real difference in pre-term birth rates between the two populations being studied?