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Random sampling without a proper map

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Jenny Dörnemann

Pediatrician, MSc student

Normal user

19 May 2011, 11:57

Dear all,

I am a pediatrician and Master Student of International Health and I am quite new in the field of nutrition.
I am preparing a survey of IYCF practices in Haiti. This will serve as a (part of a) baseline for a mother and child-health project, and it is supposed to be my Master thesis.
To prepare and design the study, I follow the manual of CARE.
Now I am stuck at the question of random sampling. We do not have any household data about the catchment area of the project. The catchment area consists of several roads with proper houses, and of several tented camps. We have a map that goes only up to street level and does not indicate the houses. On GoogleEarth we can see the houses quite well. Of the tented camps we do not have any map.

Does any of you have a practical idea how to do random sampling in this situation? What is the most feasible approach?

Kind regards
Jenny

Mary Arimond

University of California, Davis

Normal user

19 May 2011, 13:14

Here is one resource, available on-line, with a relevant section on sampling (including when accurate maps are not available): Gorstein J, Sullivan KM, Parvanta I, Begin F. Indicators and Methods for Cross-Sectional Surveys of Vitamin and Mineral Status of Populations. The Micronutrient Initiative (Ottawa) and the Centers for Disease Control and Prevention (Atlanta), May 2007. Though the title indicates micronutrient status surveys, the section on sampling is relevant for other surveys as well. Please Google the title, you should find it on-line.

Mark Myatt

Consultant Epideomiologist

Frequent user

19 May 2011, 15:29

Not sure that you really need random sampling. Some form of systematic sampling might do. Take a look at:

http://www.brixtonhealth.com/ASTRA.mov

If this suits then I can send the paper documentation.

You may find this useful:

http://www.brixtonhealth.com/urbanSampling.pdf

I hope this helps.

Jenny Dörnemann

Pediatrician, MSc student

Normal user

19 May 2011, 20:11

Dear Mark, dear Mary,

thank you so much for your input!

Mark, the epidemiologist I discussed with heavily advised against systematic sampling. She said it is too much prone to bias. So can you tell me why you think it would be ok to to it?
I very much liked the video and this method seems very feasible in the given setting. Could you send it to me in written form, too? I just think that perhaps the sample size is too big for this approach. We have a population of about (!!) 30.000 inhabitants / 8.000 households and I have calculated a sample size of 1065 children in the age 0-24 months. And from the satellite pictures not more than 10 "subdivisions" make sense. Do you still think the method in the video is feasible?

I am sorry if my questions are terribly naive.

Kind regards
Jenny

Tamsin Walters

en-net moderator

Forum moderator

20 May 2011, 09:45

From Brad Woodruff:

Dear Ms. Dörnemann:

I would recommend the survey manual published by WFP. It is available at http://www.unhcr.org/45f6abc92.html. Although there are many good survey manuals available from many organizations, they frequently recommend some variant of the "find the center/spin a pen" household selection method popularized by the Expanded Programme on Immunization back in the early 70s. Both theory and evidence indicate that this method potentially injects sampling bias into household selection. Moreover, this method also allows the survey teams doing the household selection too much leeway to decide which households to select, and this always increases the risk of sampling bias. For these reasons, I recommend against it.

In your situation, it sounds like you might be able to use Google Earth to delineate segments of the population using boundaries which can be identified on the ground, for example, roads, streams, fences, etc. The segments should be as small as possible. You then estimate the relative size of the segments, as measured by the number of households, so you can randomly select 25-30 of them using probability proportional to size. In each selected segment, you ask local informants to help you construct a list of all households currently living in that segment. Then you can use systematic random sampling or simple random sampling to select the required number of households from the list.

If you would like additional information, you can consult a web-based course on epidemiologic methods in emergencies set up by the London School of Hygiene and Tropical Medicine: http://conflict.lshtm.ac.uk/.

I hope this is useful.

Regards,
Brad Woodruff

Mark Myatt

Consultant Epideomiologist

Frequent user

7 Jun 2011, 18:44

Jenny,

Sorry for the delay ... I'm on leave.

You can use systematic sampling provided that you avoid the "gotcha" of a coincidence of your sampling interval with an aspect of population structure. If you take an even spatial sample (e.g. sample on a 10 km grid) you might find that you sample falls only in valleys or on hilltops. Take very second name in a list and you might find you select on males or only females. You can see that this sort of mistake can lead to a biased sample. Systematic samples are quite common when you don't already have good lists on which to base a simple random sample. Be careful and you should be OK.

The ASTRA sampling guide is at:

http://www.brixtonhealth.com/astraSG.pdf

Your sample size looks very large ... you must want high precision. I think that you may need to calculate your sample size allowing for a finite population. With a population of 30,000 you'd expect there to be about 8% between 0 and 2 years (2400). To estimate 50% with +/- 5% precision you'd need about n = 330 which would be less than about 500 households.

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