SADC Course in Statistics

Hierarchies of Units and non-traditional sampling approaches

(Session 18)

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Learning Objectives

By the end of this session, you will be able to

• explain the importance of recognising hierarchies of units and the need to consider sampling effort at the different stages

• understand the need for in-depth studies with a few units for qualitative enquiries

• appreciate a wide range of non-traditional sampling approaches that may be used in the overall sampling scheme

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Hierarchies of units

• Information exists at various levels & these need to be acknowledged, analysed and used in the sampling scheme, e.g. farming household, village, parish, sub-county, county, district, Uganda.

• For example, for research in one district,– Sub-county = primary unit– Parish = secondary unit– farming household = ultimate sampling unit.

• Often referred to as “multi-stage” sampling, but this suggests successive time points – we prefer “hierarchical”

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Disposition of resourcesConsider for example Mukono district in Uganda

• It has 17 sub-counties

• How should sub-counties be sampled?

• In each selected sub-county should sampling be

– 2 parishes x 20 h/holds in each?

– 8 parishes x 5 h/holds each?

• Can results be generalised to the district with the scheme adopted?

• Use 8, to predict and confirm makes more sense!

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Sampling up hierarchies

• Top level units “well known”, characterised in many ways. Total number of units is likely to be small.

• “Random” sampling often meaningless, as likely to be unrepresentative in face of large amounts of information about units

(&/or politically unacceptable)

• Units at top level are often chosen purposively using knowledge about these units (see slide 11 for an example)

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Sampling down hierarchies

• Ultimate units need to be anonymous to research users who need assurance of purposeful, well-defined, objective selection.

• Must avoid selections based on vaguely-defined variables that make respondent selections different in each location.

• There is a need to incorporate an element of random sampling at some stage, at least at the final level of sampling.

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Objectives in hierarchies

• Need to think about sample size and information use (e.g. reporting and uptake) at each level of hierarchy.

• Need to ensure sample selection is related closely to research objectives and in turn to the recommendation domain for which results are intended.

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Numericalness of objectives

• Main objective in statistical sampling texts is accurate numerical estimation: not usually the aim in many studies having qualitative aims!

• In on-farm studies, do not expect new technologies to apply equally well to all farmers

• Which technologies are most effective for which types of farmers is likely to be an important objective.

• Estimation of numerical quantities less relevant here, so use common-sense based approaches

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Comparison as objective

• Rather than quantitative estimation [or a qualitative description], we often want to compare groups, with each other – implies different priorities as to representativeness…

• Population 80:20 split implies estimation best based on 80:20 sample split BUT

comparison best based on 50:50 sample split so both things compared are equally well understood.

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Many comparisons

Example :

• some areas have good, some bad transport;

• some have good environmental conditions, some have bad conditions;

• some have, some have not started a District Poverty Initiative Programme.

Does study cover all combinations??

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Two of everything!

Good environmental conditions

Bad environmental conditions

Trans-port

DPIP + DPIP - DPIP + DPIP -

Good --- Include Include ---

Poor Include --- --- Include

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Combining or splitting studies

• Combine a short questionnaire-style study with more intensive qualitative and/or experimental follow-up

• Sampling in the 1st phase may help determine samples for choice of 2nd phase sites – “Table top” sampling plan – as shown in next slide

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People sampled within each of a number of communities

Subsample for deeper

research

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Succession of studies

• If we can have a succession of studies linked by sampling scheme, ‘read-through’ may be useful, if respondent burden permits

• The next slide gives a graphical illustration

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Vertical links = in-depth studiesHorizontal at top = main survey studyHorizontals below = sectoral studies

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Segmenting a study

• Core items done with all respondents, others modularised and done with separate sub-samples – reduces respondent burden

• Or studies separately undertaken but linked by common core items

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Main and sub-theme coverageof segmented study

Core Questions

Module 1

Module 2

Module 3

All sections of respondent sample

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Targeting special groups

• e.g. how do we sample the poor?

Screening methods OK for target groups common in population

• Snowball sampling explores rarer, mutually-aware, subgroups

• Adaptive sampling - adjust effort where more are found – theory not well developed, currently mainly suits immobile populations.

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Replicated sampling

• Entire structure of study repeated in miniature, n times

• In quantitative studies, can incorporate any number of complications (same ones in each replicate) then treat overall summaries from the replications like a random sample

• Analogy could work for qualitative research which seeks reproducibility / generalisability

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References• SSC (2000) Some Basic Ideas of Sampling. Statistical

Guidelines Series supporting DFID Natural Resources Projects, Statistical Services Centre, The University of Reading, UK. Available at

www.reading.ac.uk/ssc/publications/guides.html

• Wilson, I.M. (2005) Some Practical Sampling Procedures for Development Research, pp. 37-51 in Holland, J.D. and Campbell, J. (editors) (2005) Methods in Development Research ; Combining Qualitative and Quantitative Approaches, ITDG Publishing. ISBN 1 85339 572 2

• Wilson, I.M. and E.F. Allan (2004) Sampling Concepts In: Stern, R.D., Coe, R., Allan, E. and Dale, I.C.(ed.s) Statistical Good Practice for Natural Resources Research. CAB International, Wallingford, U.K. pp.65 – 86

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Practical work follows …