Handicap International experience in Afghanistan - CBM

Survey MethodologyFrom Sampling Requirements to Field RealitiesSECTION1The methodology that was followed for the NDSA was a random, probability proportional to size (PPS),three step sampling. A number of steps were taken in order to ensure that the sample selected wasrepresentative of the Afghan population. Following the defined sampling procedure helped ensure that theconclusions made for the selected sample were valid for the entire Afghan population.Why Select a Sample?Choosing a representative sample has many advantages in comparison to other types of survey or a census,for various reasons:• It is close to impossible to include everyone in terms of time, distances and resources (humanand financial).• Including everyone is not necessary: statistically the same results can be obtained from a small part ofthe whole.• Using a sample also allows an in-depth analysis of specific issues, which a census does not.Any PPS sampling frame is based on two principles:• Everyone has an equal chance of being included in the sample.• The sample selected has the characteristics of the parent population in order to avoid bias.Defining the Sample SizeA national survey on disability requires a sound sampling methodology that allows the establishment of validprevalence rates at the national level. As a result the conclusions made for the sample would be relevant atboth, the national and sub-national levels. Our partners at Johns Hopkins Bloomberg School of Public Healthcarried out the necessary calculations and defined the number of households that needed to be surveyed inorder to obtain the rate of prevalence with the wanted accuracy.These calculations gave us the number of households according to three factors;• The rate of prevalence of disability, estimated at 6%;• The confidence interval;• The level of precision chosen.The representative sample selected consisted of 5250 households 1 , which represented around 39,000persons, with a confidence interval of 95% and a precision of 15%.Following the Sampling StagesThe NDSA is a probability proportional to size (PPS), three-stage random sample survey. This means thatat each stage of the sampling process each household had an equal chance of being selected. This in1For the NDSA, the household is a group of people who may or may not be related by blood or marriage, who live under the same roof orwithin a same compound, who take their meals together or in small groups (one kitchen in the house), who put together part or all of theirincome for the needs of the group and who are dependant on the authority of the head of the household for all the expenses entailed. Youmay come across ‘uni-personal’ households that consist only of one person: for our survey this concerns the situation where the intervieweelives alone. The definition adopted is similar to the one adopted by the CSO for the pre-census of 2004, by NRVA in 2003 and 2005 andby Johns Hopkins household surveys.