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Non Probability Samples

Also: Sampling at Service Sites (SSS)

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Non Probability Samples: BACKGROUND

Non-probability sampling differs from probability sampling in that it does not use random selection. The latter ensures that all units being studied (i.e., members of a population) have an equal opportunity of being selected for participation. While often considered the most rigorous sampling method, probability samples may not be feasible in certain research contexts due to limited time, resources or other constraints. Non-probability samples can be useful in such circumstances and fall into two main categories: convenience and purposive sampling. Purposive sampling is more commonly used and involves targeting social groups, experts or key informants. Results of non-probability techniques are limited, however, as samples are not representative and confidence intervals cannot be generated.

Sampling at Service Sites (SSS) is an adaptation of non-probability sampling for measuring maternal mortality. Developed by Immpact, it samples respondents at service or other sites (such as antenatal care sites, child health services, or at market places) and asks sisterhood method questions. SSS uses high coverage sites, data on respondent characteristics and reliance of reporting on siblings to overcome some biases.

Identification of death

Adult respondent reports death using either the indirect or direct Sisterhood Methods.

Ascertainment of maternal/pregnancy related status

Adult respondent reports deaths (and sex and age at death) among all his/her brothers and sisters born to the same mother in response to sibling survivorship questions and direct sisterhood method questions.

Advantages:

Limitations:

Measurement requirements:

Data as required by use of the indirect or direct sisterhood methods. See above.

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Oxford Journals - International Journal of Epidemiology Danel, I; Graham, W; Stupp, P; Castillo, P. (1996) Applying the sisterhood method for estimating maternal mortality to a health facility-based sample: a comparison with results from a household-based sample. International Journal of Epidemiology, 25(5): 1017-22.

National Statistical Service, Australia Doherty, M. (1994) Probability versus Non-Probability Sampling in Sample Surveys, The New Zealand Statistics Review, March 1994 issue: 21-8.

Indian Journal of Medical Research Singh, P; Pandey, A; Aggarwal, A. (2007) House-to-house survey vs. snowball technique for capturing maternal deaths in India: A search for a cost-effective method. Indian Journal of Medical Research, 125: 550-6.

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Immpact SSS Tool: Calculator for number of survey respondents required

Immpact Immpact, University of Aberdeen (2007) Sampling at Service Sites (SSS), Module 4, Tool 1. In: Immpact Toolkit: A guide and tools for maternal mortality programme assessment. Aberdeen: University of Aberdeen.

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