Copyright 2010, The World Bank Group. All Rights Reserved. Part 1 Sample Design Produced in Collaboration between World Bank Institute and the Development Data Group (DECDG)

Copyright 2010, The World Bank Group. All Rights Reserved. Sampling and Total Survey Error Impractical to survey entire populations Samples are drawn, data collected, and estimates are made that won’t equal true population values Sampling Error is error arising solely from the sampling process (measure: variance) Surveys are also subject to biases from nonsampling errors such as nonresponse

Copyright 2010, The World Bank Group. All Rights Reserved. Key Sample Design Issues Probability based to allow valid analysis –Unbiased estimates (nearly) with 100% response –Ability to measure variance Frame that covers target population well –Ability to identify and access chosen units Analytic needs (What estimates are needed?) –How good do variances have to be? Geographic or other stratification

Copyright 2010, The World Bank Group. All Rights Reserved. Household Frame Worldwide, labor force surveys use samples of households (HHs) or housing units Target population of adults that may be in the labor force Difficult to sample persons directly But fairly easy to sample households –Some countries have HH lists –Areas frames used when no list

Copyright 2010, The World Bank Group. All Rights Reserved. Area Frames An ideal frame for a household survey: Is 100% Complete (includes all HHs) Has 0% destroyed/empty HHs Has 0% units that are not HHs at all Has 0% chance of duplicating a HH Has 100% ability to locate a sampled HH Area Frames are very good in this regard and are widely used (see diagram)

Copyright 2010, The World Bank Group. All Rights Reserved. 6 HHs missing from frame (Undercoverage) Target Population *extra HHs in frame Frame *Survey work can identify extra units in the sample so that you don’t use them in estimates (avoids overcoverage). HH Frame Coverage Target Population Covered by Frame

Copyright 2010, The World Bank Group. All Rights Reserved. 2-Stage Area Sampling First-Stage Sample of Areas Define areas 100% covering a country –Can be large or small –Enumeration Areas (EAs) of HH Information on each EA from last census is available –Select a probability sample of areas Second-Stage Sample of Households List HHs only in the 1 st -stage sampled areas –Select a probability sample of the listed HH

Copyright 2010, The World Bank Group. All Rights Reserved. Why 2-Stage Area Sampling? Area frames are typically used if: No list of households or housing units OR there may be a list that is not up-to-date (misses new construction) 1 st -stage geographic units that contain the households are readily available (such as enumeration areas)

Copyright 2010, The World Bank Group. All Rights Reserved. Why 2-Stage Area Sampling? Area frames: Areas: 100% complete with 0% duplication Up-to-date if households in the stage 1 units are re-listed Avoid difficulties of: –Creating a list –Supplementing list with new construction

Copyright 2010, The World Bank Group. All Rights Reserved. List Households or Housing Units? After listing HHs, time is needed to sample the households to be surveyed Interviewers return to the field after the 2 nd -stage sampling operation is completed It is necessary that the interviewer be given enough information to re-locate a sampled household

Copyright 2010, The World Bank Group. All Rights Reserved. List Households or Housing Units? Housing Unit (HU) is a physical address –List by observation –Find later from an address (number & street) HHs often listed in more rural countries –Atypical HUs, and problems finding HUs –More intense HH listing, obtain head of household –Defined using family ties & living arrangements –Locate later by head of household

Copyright 2010, The World Bank Group. All Rights Reserved. Use of Households (HHs) HUs can be difficult to identify in some areas and many addresses difficult to locate if they are sampled. Villages? Farms? Cities? Towns? HHs identified by head of household can readily be found after sampling. Up-to-date if sampling and surveying occur shortly after the listing operation. 12

Copyright 2010, The World Bank Group. All Rights Reserved. A Typical 2-Stage Design Major Stratification by Geography –Subnational data desired (states, regions, etc.) Minimum of about 300 responding HH in each –Administrative convenience Stage 1 sample of EAs, systematic PPES –PPES = Probability Proportional to Estimated Size Stage 2 sample of HHs, systematic –List HHs in the EAs selected in the first stage –Select a fixed number of HHs per EA (ex: 10 HHs)

Copyright 2010, The World Bank Group. All Rights Reserved. Why sample EAs PPES? Enumeration areas are relatively small (ex: HHs) But they differ in size –That makes Probability Proportional to Estimates Size sampling efficient EA size from the last census can be used –“old” so size is estimated and not actual Size should be related to the labor force –# HHs, total population, # adults, etc.

Copyright 2010, The World Bank Group. All Rights Reserved. Example: Sampling EAs PPES Small geographic area of a country with only 7 EAs and 2,000 estimated HHs  sample 2 Introduce concept of Sampling Interval (SI) –In this example SI = 1,000 = 2,000/2 the total # estimated HHs divided by the # sample Eas Random number (rn) determines selections –rn is between 0 and 1 (ex.423) –1 st selection compute: SI * rn = 1,000 *.423 = 423 –2 nd selection compute SI * rn + SI = 423+1,000 = 1,423

Copyright 2010, The World Bank Group. All Rights Reserved. Example: Sampling EAs PPES EA HHest CumHH choose 2 EAs marked by * EA EA * The first EA with CumHH EA greater than 423 is EA2 EA EA * The first EA with CumHH EA greater than 1,423 is EA5 EA

Copyright 2010, The World Bank Group. All Rights Reserved. Example: Sampling EAs PPES HHest is the estimated size of each EA –The # HH from the last census CumHH column cumulates Hhest 2 EAs selected are the ones that first exceed the calculated numbers 423 and 1,423 –1st selection: EA2 CumHH of 550 exceeds 423 –2 nd selection: EA5 CumHH of 1,500 exceeds 1,423

Copyright 2010, The World Bank Group. All Rights Reserved. Example: EA Probabilities EA Probability of selection: HHest/SI The EA sizes are estimated –but the probabilities are valid –probabilities sum to 2, the sample size –As actual EA sizes become known for the sampled EAs, DO NOT change the probabilities Save the probabilities of sampled EAs

Copyright 2010, The World Bank Group. All Rights Reserved. Example: EA Probabilities EA HHest PPES probabilities EA = 350/1000 EA2* = 200/1000  save! EA = 400/1000 EA = 250/1000 EA5* = 300/1000  save! EA = 350/1000 EA = 150/

Copyright 2010, The World Bank Group. All Rights Reserved. Example: EA Weights Weight -- inverse of a sampling probability –In this case only of the EA probability EA2 probability.2 weight 5 = 1/.2 EA5 probability.3 weight = 1/.3 Simple check of EA probabilities –HHest * weight should equal the SI EA2 HHest *5 = 1,000 = SI EA5 HHest * = 1,000 = SI

Copyright 2010, The World Bank Group. All Rights Reserved. Example: List & Sample HHs Listing only needed for sampled EA2 & EA5 Select fixed sample n = 10 HH in each EA –Common range for n is HH per EA Sample HHs systematically from an EA –HHs not random, but periodicity is not a problem –HHs are listed in simple geographic order –HH sort by economic variables can add efficiency (rich/poor? urban/rural?)

Copyright 2010, The World Bank Group. All Rights Reserved. Example: Sample HH from EAs EA HHest PPES HH n HHprob EA EA HH is the actual number of HHs listed Probabilities of selection are based on HH ED2 HHprob = n/HH = 10/210 = ED5 HHprob = n/HH = 10/280 =

Copyright 2010, The World Bank Group. All Rights Reserved. Example: Sample HH from EA2 Calculate a sampling interval (SI) for EA2 –SI = HH/n = 210/10 = 21 –Choose a random start from 1-21, ex: rs = 12 The HHs are listed 1, 2, 3, 4, 5, …, 210 Choose rs, rs+SI, rs+2SI, rs+3SI, …, rs+9SI 12 th 33 rd 54 th 75 th 96 th 117 th 138 th 159 th 180 th 201 st

Copyright 2010, The World Bank Group. All Rights Reserved. Example: Sample HH from EA5 Calculate a sampling interval for EA5 –SI = HH/n = 280/10 = 28 –Choose a random start from 1-28, ex: rs = 6 The HHs are listed 1, 2, 3, 4, 5, …, 280 Choose rs, rs+SI, rs+2SI, rs+3SI, …, rs+9SI 6 th 34 th 62 nd 90 th 118 th 146 th 174 th 202 nd 230 th 258 th

Copyright 2010, The World Bank Group. All Rights Reserved. Example: Total 2-Stage Probability Multiply the probabilities from each stage EA HHest PPES HH n HHprob EA EA Multiply the probabilities PPES * HHprob and the inverse is the weight for EA2.200  = weight 105 for EA5.300  = weight

Copyright 2010, The World Bank Group. All Rights Reserved. Self Weighting Samples Probabilities and weights in the example are different for EA2 and EA5 Can modify HH sampling to make the probabilities and weights equal (almost) Samples where all weights are equal (or all weights in a stratum) are called self-weighting samples. Small gains in efficiency

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