MEASURES OF IMPACT

INTRODUCTION TO PART II: MEASURES OF IMPACT

Measure of association, such as the risk ratio and odds ratio, are used to address hypotheses aimed to determine the strength of association between exposure and disease.  For example, how strong is the association between smoking and stomach cancer? 

In contrast, measures of impact relate to questions such as: How much of the disease that occurs can be attributed to a certain exposure? How many people would need to alter their exposure to prevent one case from occurring?  Some commonly used measures of potential impact include:

– attributable risk (AR), or risk difference (RD) 

– population attributable risk (PAR)

– number needed to treat (NNT) to prevent one case. 

– etiologic fraction in the exposed (EFe) or attributable fraction (AF)

– etiologic fraction in the population (EFp), or population attributable fraction (PAF)

SECTION B

Peri-conceptional use of multivitamins containing folic acid can reduce a woman’s risk of having a baby with a neural-tube defect (NTD). As part of a public health campaign conducted from 1993 to 1995 in an area of China with high rates of neural-tube defects (the northern region) and one with low rates (the southern region), Berry and colleagues (1999) evaluated the outcomes of pregnancy in women who were asked to take a pill containing 400 mg of folic acid daily from the time of their premarital examination until the end of their first trimester of pregnancy.  The following is a quote from the study results:

“Among the fetuses or infants of 130,142 women who took folic acid at any time before or during pregnancy and 117,689 women who had not taken folic acid, we identified 102 and 173, respectively, with neural-tube defects. Among the fetuses or infants of women who registered before their last menstrual period and who did not take any folic acid, the rates of neural-tube defects were 4.8 per 1000 pregnancies of at least 20 weeks’ gestation in the northern region and 1.0 per 1000 in the southern region.  Among the fetuses or infants of the women with peri-conceptional use of folic acid, the rates were 1.0 per 1000 in the northern region and 0.6 per 1000 in the southern region.”

Berry RJ, Li Z, Erickson JD, Li S, et al. Prevention of neural-tube defects with folic acid in China. N Engl J Med. 1999;341(20):1485-90

B.1.       Which measure of disease frequency is reported in this study?

B.2.   Name, calculate and interpret all appropriate measures of association between prenatal folic acid use and NTD in this study ?