The goal of a study by Crosignani et al. (A-28) was to assess the effect of road traffic exhaust on the risk of childhood leukemia. They studied 120 children in Northern Italy identified through a population-based cancer registry (cases). Four controls per case, matched by age and gender, were sampled from population files. The researchers used a diffusion model of benzene to estimate exposure to traffic exhaust. Compared to children whose homes were not exposed to road traffic emissions, the rate of childhood leukemia was significantly higher for heavily exposed children.

Characterize this study as to whether it is observational, prospective, or retrospective. Describe the variables as to whether they are continuous, discrete, quantitative, qualitative, a risk factor, or a confounding variable. Explain the meaning of the reported results. What are your conclusions based on the given information?