A recent cohort study found an increased risk of childhood leukemia in children born to mothers who had previously chosen to use hormonal contraception (Hargreave et al. 2018). The study also noted that use of such contraception was more common for children whose parents were younger, had lower education levels, and whose mothers who smoked during pregnancy, than for children whose mothers did not use contraception.

a. Is this an observational or experimental study?

b. Can we conclude that previous use of hormonal contraception by mothers is a cause of higher rates of leukemia in their children? Why or why not?

c. What do we call the variables “parent age,” “parent education level,” and “smoking” if they distort the association between rates of leukemia in children of mothers who recently used hormonal contraception?