The study is nested in the Nairobi Urban Health and Demographic Surveillance System (NUHDSS), which collects routine health and demographic data from about 76,094 people in 29,900 households (as at the end of 2009) in the two slums ([@bibr2-0165025413479299]). During the first wave of data collection, about 4,057 youths were interviewed using a structured interviewer-administered questionnaire between October 2007 and June 2008. In the second wave (March 2009--August 2009), 2,527 youths were re-interviewed and 1,629 youths were re-interviewed in the third wave (April 2010--August 2010).

The questionnaire included questions covering social demographic characteristics (e.g., independent housing and schooling), and living arrangements, as well as other psychosocial and behavioral factors. The questionnaire was developed and reviewed by a team of experts in youth issues and was pilot tested among anot correspond to the contour WG, is inserted in the movement path K. Machining accuracy is increased and machining time is reduced at corners because needless movement is omitted from the corners.
The discharge gap is the distance at which the discharge is generated between the wire electrode and the workpiece. The wire electrode applies an electrical discharge to the workpiece from a distance of the discharge gap. The discharge gap varies according to the machining condition, such as voltage. The higher the voltage is, the larger the discharge gap becomes. The lower the voltage is, the smaller the discharge gap becomes. Generally, the discharge gap is larger for rough machining, and smaller for fine machining.
In the Numerical Control program, a block is a unit of movement along the