b. Sampling scheme The proposed field area for the study is Mirpur, an "auxiliary Thana" in the Dhaka Metropolitan City Corporation with a total population of 5,580,000 (according to the Census 2001, although it may have increased by a further 20% to 30% since then). Mirpur has 16 "Wards" which are the smallest urban administrative unit each of which has a local government authority called a "Ward Council". The average population of a "Ward" is 350,000 people. Below the "Wards" are "Mahallas" which are the lowest urban geographic unit with identifiable boundaries. On average there are five "Mahallas" per "Ward" with an average population of 70,000 people. The unit of randomization for the trial will be Mahallas. In Mirpur there will be a total of 80 Mahallas, which will form the sampling frame for selecting the clusters in the trial. The list of these Mahallas is available from the Dhaka Metropolitan City Corporation.