An abstract in a journal article is a short synopsis of the study. The following abstract is from an article in the British Medical Journal (BMJ). The article uses relative risk in the analysis, which is common in health-related studies. Relative risk represents how many times more likely an individual is to have some condition when compared to an individual who does not have the condition. Childhood Cancer in Relation to Distance from High Voltage Power Lines in England and Wales: A Case-Control Study Objective To determine whether there is an association

between distance of home address at birth from high voltage power lines and the incidence of leukemia and other cancers in children in England and Wales.

Design Case-control study.

Setting Cancer registry and National Grid records.

Subjects Records of 29,081 children with cancer, including 9,700 with leukemia. Children were aged 0–14 years and born in England and Wales, 1962–95. Controls were individually matched for sex, approximate date of birth, and birth registration district. No active participation was required.

Main outcome measures Distance from home address at birth to the nearest high voltage overhead power line in existence at the time.

Results Compared with those who lived 7600 µ from a line at birth, children who lived within 200 µ had a relative risk of leukemia of 1.69 (95% confidence interval 1.13 to 2.53); those born between 200 and 600 µ had a relative risk of 1.23 (1.02 to 1.49). There was a significant (P-value < 0.01) trend in risk in relation to the reciprocal of distance from the line. No excess risk in relation to proximity to lines was found for other childhood cancers.

Conclusions There is an association between childhood leukemia and proximity of home address at birth to high voltage power lines, and the apparent risk extends to a greater distance than would have been expected from previous studies. About 4% of children in England and Wales live within 600 µ of high voltage lines at birth. If the association is causal, about 1% of childhood leukemia in England and Wales would be attributable to these lines, though this estimate has considerable statistical uncertainty. There is no accepted biological mechanism to explain the epidemiological results; indeed, the relation may be due to chance or confounding.

(a) Describe the statistical process. Include an interpretation of the confidence interval and P-value.

(b) Why is this study an observational study? What is a case-control study? Explain why a research objective such as this does not lend itself to a designed experiment.