Final answer:
The relative risk (RR) is used to quantify the association between illicit drug use and giving birth to a child with NTDs. A value greater than 1 indicates an increased risk, while a value less than 1 indicates a reduced risk.
Step-by-step explanation:
The relative risk (RR) is a measure used in epidemiology to quantify the association between two variables, in this case, illicit drug use and giving birth to a child with NTDs (neural tube defects). To calculate the RR, we divide the incidence of the outcome (giving birth to a child with NTDs) in the exposed group (drug use group) by the incidence of the outcome in the non-exposed group (no drug use group).
To interpret the RR, a value greater than 1 indicates an increased risk of giving birth to a child with NTDs in the drug use group compared to the no drug use group. A value less than 1 indicates a reduced risk, and a value equal to 1 indicates no association between drug use and NTDs.