Day care centers expose children to a wider variety of germs than the children would be exposed to if they stayed at home more often. This has the obvious downside of more frequent colds and other illnesses, but it also serves to challenge the immune system of children at a critical stage in their development. A study by Gilham et al. (2005) tested whether social activity outside the house in young children affected their probability of later developing the white blood cell disease acute lymphoblastic leukemia (ALL), the most common cancer of children. They compared 1272 children with ALL to 6238 children without ALL. Of the ALL kids, 1020 had significant social activity outside the home (including day care) when younger. Of the kids without ALL, 5343 had significant social activity outside the home. The rest of the children in both groups lacked regular contact with children who were not in their immediate families.

a. Is this an experimental or observational study?

b. What are the proportions of children with significant social activity in children with and without ALL?

c. What is the odds ratio for ALL, comparing the groups with and without significant social activity?

d. What is the 95% confidence interval for this odds ratio?

e. Does this confidence interval indicate that amount of social activity is associated with ALL? If so, did the children with more social activity have a higher or lower occurrence of ALL?

f. The researchers interpreted their study results in terms of the differing immune system exposure of the children, but gave several alternative explanations for the pattern. Can you think of any possible confounding variables?