2.4.1. Primary Outcome
The primary outcome for this analysis was child exposure to tobacco smoke, measured by hair nicotine. Child hair nicotine was chosen because it reflects long-term exposure (approximately 1 cm of hair growth/month) [We analyzed TSE in two ways. First, we assessed TSE as a binary variable indicating “exposed” (yes, above the limit of detection) versus “unexposed” (no, below the limit of detection). This approach is useful because there is no safe level of exposure to tobacco smoke, so any exposure is important, and also because it provides information on the proportion exposed, which is easily understandable. Second, we assessed TSE as a continuous variable based on the levels of hair nicotine and used the natural log transformation because of the known non-normal distribution of hair nicotine. Hair nicotine was analyzed in ten separate batches over the course of the research.