Background In the small island nation of Epidoria a team of

Background - In the small island nation of Epidoria, a team of reproductive epidemiologists has been studying the relationship between very low birth weight and risk of cognitive, motor, and behavioral problems. Five years ago these investigators initiated a study. Using birth certificate files and delivery room entry logs, these investigators attempted to identify all full-term births in Epidoria over a 6-month period. The investigators enrolled all low birth weight babies and a representative sample of normal birth weight babies into their study. The investigators then examined the children every year until age 3 years. During the last examination, the investigators administered a standardized developmental screening test to assess personal-social, language, and motor-adaptive skills. Based on this test, the investigators classified the children into two groups: normal development and delayed development.

The results from the study were:

What kind of a study is this? What is the exposure under study? What is the outcome under study?

Calculate the crude cumulative incidence ratio (relative risk) .

Background (cont.) - To account for the possibility that environmental lead exposure might confound the relationship between birth weight and developmental status, blood lead levels were determined from blood samples collected at the age 3-year visit. Elevated lead levels (> 10 micrograms/ dL) were found in 173 of the low birth weight children (88 of whom had delayed development according to their screening test). Elevated lead levels were also found in 72 of the normal birth weight children (24 of whom had delayed development).

Carry out a stratified analysis of birth weight and developmental delay, controlling for blood lead level. Create 2x2 tables for each stratum of lead level and estimate the relative risk for each stratum. Comparing the findings from the crude and stratified analysis, is there a suggestion of confounding by lead exposure in this example (2 points)?

Is there evidence of an association between blood lead level and the primary exposure variable in this study (hint create a 2x2 table for blood lead level and exposure)?

Is there evidence of an association between blood lead level and the outcome in this study?

Based on all of this information would you judge blood lead level to be a confounder of the association between birth weight and delayed development? Explain your answer.

What are two changes in the study design would have avoided the potential confounding effects of blood lead level? What are the advantages and disadvantages of these alternatives?

Solution