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Figure 1 | Respiratory Research

Figure 1

From: Extent of exposure to environmental tobacco smoke (ETS) and its dose-response relation to respiratory health among adults

Figure 1

The overall sampling scheme of the Aleppo Household Survey. In the 1st step the target population was divided into two strata, formal and informal zones (where residential areas are build illegally or on a land not designated for housing). In the next step residential neighborhoods were selected with PPS, and within selected neighborhoods households and one adult within each were selected with equal probability.

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