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# WHO EPI two stage cluster survey method Vis-à-vis DHS or MICS approach

This question was posted the Assessment and Surveillance forum area and has 7 replies.

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### Mark Myatt

Frequent user

7 Dec 2009, 12:07

First ... sorry for not replying sooner. I usually reply after an e-mail notificaton of a posting and either I did not get one or I did and it got overlooked. ENN are looking into this. Anyway ... There a couple of "problems" that you have to address: Multiple indicators : It seems to me that the bulk of the indicators mentioned are simple proportions or percentages. What you need to do is make a list of each indicator and, for each indicator, write down the expected level (if you do not have any good idea then use 50%) and the desired level of precision (i.e. the width of the 95% confidence interval). When you have done this you should use a sample size calulator such as SampleXS: http://www.brixtonhealth.com/samplexs.html Or GNU sampsize, which is available online at: http://sampsize.sourceforge.net/iface/index.html And calculate the required sample size for each indicator. The largest sample size that you calculate here is the the smallest sample size that will yield the desired precision for [u]all[/u] of your indicators. It is, unfortunately, not as simple as that. The indicators apply to different units. For example, the nutritional anthropormetry indicator applies to individual chidlren but a sanitation indicator may apply to a household. You have to account for this when you calculate sample sizes. I will give an example with three indicators: GAM : Expected prevalence = 12%, desired precision = +/- 3% EPI : Expected coverage proportion = 70%, desired precision = +/- 10% Safe disposal of faeces (SDF) : Expected proportion safe = 50%, precision = +/- 10% When I use GNU sampsize for these I get 451, 81, and 97 respectively. The problem here is that the indicators apply to different units: GAM : Children aged 6 - 59 months EPI : Children aged 6 - 24 months SDF : Households You have to find some way of "standardising". The easiest way is to work with households and express the sample size in terms of the number of households required. If we assume that we will find 1.25 children aged between 6 and 59 months in a sampled household then we would need to sample 451 / 1.25 = 361 households to find 451 children aged between 6 and 59 months. If we expect to find 0.25 children aged between 6 and 24 months in a household then we would need to sample 81 / 0.25 = 234 households to find 81 chidren ages between 6 and 24 months. And, of course, you need to sample 97 households to find 97 households. So the sample sizes expressed in numbers of households is: GAM : 361 EPI : 234 SDF : 97 Again, it is not as easy as that. For a cluster sampled survey we have a design effect (DEFF) to consider. This will be different for different indicators. It will be particulary high for anything that tends to cluster spatially (either within or between villages) such as infectious diseases or program coverage. You need to make a guess at these and multiply the calculated sample size by the expected DEFF. If we assume that GAM is not very clustered (DEFF = 1.5) and that EPI and SDF are likely to be more clustered (e.g. DEFF = 3 and DEFF = 2 respectively) then our sample sizes are now: GAM : 361 * 1.5 = 541 EPI : 234 * 3 = 702 SDF : 97 * 2 = 194 If you use a cluster sampled apprach then you will need about 30 clusters (do not go much below this). To calculate the within-cluster sample size you shoudl divide the largest sample size by the number of clusters. In our example this will be 702 / 30 = 24 households. It is common practice with sample size calculations to round up the results of calculations. This may seem to be a compicated procedure but should not present problems if you take it step-by-step. The "homogeneity assumption" : The main problem with the desigh you propose is that it provides a single estimate for an indicator. This is OK as long as it makes sense to have a single estimate. As a survey area gets larger the chances of a single estimate being meaningful decreases. For example, imagine that you cover two districts in a single survey. One district has an active EPI program and the other does not. If the true EPI coverage in the first district is 80% and the true EPI coverage in the second district is 30% then your survey might tell you that EPI coverage is about 55%. You would conclude that EPI coverage was poor everywhere but the truth is that in one disctrict it is pretty good while in the other district it is very bad. Also, neither district has an EPI coverage even close to 55%. IMO, such an estimate applies nowhere. [u]A survey that produces misleading results is worse than having no survey at all.[/u] What you will see in this context is a large design effect and a very wide 95% confidence interval. Not very useful. In the situation that you describe I would be very wary of doing a wide-area survey that yielded a single estimate for the wide area. Stratification (as the term is used in MICS) is one approach but to produce useful results (e.g. per-district results) you would really need to do a full sample size survey in each area. Another form of stratification that might be more useful is spatial stratitification. With this the area is divided into a set of small areas and a sample is taken from each. The trick is to make the small-area sample representative and make clever use of the data you collect. Such methods have been used for estimating CMAM program coverage and for the Myanmar Periodic Review (which uses hexagonal / triangular areas and reuses data to effcetively triple the small area saple size for free). With small samples you may need to classify (e.g. EPI < 50%, between 50% and 80%, or > 80%) rather than estimate indicator proportions. In short ... you could do a MICS type survey but you should be aware that it may yield misleading results. You should be aware that not everyone shares my poor opinion of survey designs like the MICS. I suggest that the forum administrator make a direct request to a responsible person in UNICEF for their opinions on this. I hope this helps.

### Mark Myatt

Frequent user

7 Dec 2009, 16:17

Also, MIKe Golden might have some interesting observations on this problem.

### Michael Golden

retired

Normal user

7 Dec 2009, 17:17

It is not so easy to comment: the questioner is anonymous and we have no idea where the survey is to be carried out (Urban/Rurual; emergency/stable: Asia/Africa, etc). However, in general I agree with all that Mark has said. BUT most of all, there is no indication of the OBJECTIVES of the survey. The objectives determine the design and decision making - and must be clear and definitive at the outset. There is no point in doing a survey for survey sake or just to get some vague idea of what is happening. Wide area averages can be useful for central planning of resouce procurement and allocation - but as Mark says - they do not give any indication of how or where resouces should be deployed. And the lead in time often makes the survey redundant by the time it is analysed and reported - do not forget the very major seasonality in most countries. I would add to Mark's remarks that each quesion that is added to a survey degrades the quality of all the information. It is well known that if an interview takes more than about 20mins then the respondent gives any information that they think will satisfy the questioner - and the results of the survey are almost useless. Sophistocated analysis does not make wrong information useful. When I read the quesion I said to myself- I would never ever embark on such a survey - and if there are the indicated budget and time constraints - I question if it is worth doing this survey at all! I for one would have major reservations about the results and donors and governments should as well. All these questions need to be clear before one reaches the technical decisions about sample size and procedures - these can be found in most survey manuals - SMART has all the procedures and calculations incorporated. I would not embark on such a survey without the leader of the survey being experienced and knowledgeable about these technical matters. In other words - if you have to ask these questions perhaps you should not be doing the survey?

### Mark Myatt

Frequent user

10 Dec 2009, 11:58

Just to second Mike's comments : (1) DHS and MICS are useful for central planning. My experience with using both DHS and MICS data leads me to the opinion that DHS data is more standardised and of better quality. I know that a lot of effort has gone into standard indicator sets for MICS and the situation is improving. (2) The time elment is important. Remember that a cross-sectional survey yield a snapshot or limited precision. Some indicators can change fast. (3) I absolutely agree with the "less is more" statement about dataset width and data quality. One of the major strengths of the SMART method (that Mike nursed to fruition) is that it doesn't attempt to stuff a huge number of indicators into a single survey. I know the temptation is to say "We are surveying so we might as well ask this and ask that. What's the harm?". Unfortunately, it is not as simple as that. I have a rule of thumb ... if you have more than 20 variables in a survey then you are asking for trouble. That is 20 variables not 20 indicators. Think of an indicator such as GAM. You need to collect cluster number, age, sex, weight, height, oedema, and MUAC. That is seven variables for one indicator. Another rule of thumb (which contradicts the previous one somewhat - that is the nature or rules of thumb) is "one question per survey". In this rule "question" refers to a topic. So a nutritional anthropometry survey addresses both GAM and SAM and an EPI survey addresses vacciantion status for each vaccine in the MoH "basket". I have been involved with some MICS-style surveys with large questionnaires and the results have not been good. Mike's "20 minute" rule is a good one. I agree with Mike ... I would be very wary of attempting such a survey. You might want to do a triage exercise and split your indicators into (1) "must have", (2) "nice to have", and (3) "not useful at this time". Then take all in (1) if there is room ... if not then rank and take the highest ranked. If you have room then rank (2) by importances and take the most highly ranks ones provided that you can obey the "20 variables" or "20 minutes" rules.

### Mark Myatt

Frequent user

10 Dec 2009, 17:52

Thank you for your kind comments. Now to speak frankly ... The stated objective "to determine the need for an immediate and long term response" is at odds with "to understand the causal factors". Indeed, cross-sectional surveys are notoriously very poor at investigating associations (let alone causal factors) and typically suffer from very low power. Think of it this way ... your survey has a sample size of 384 (this will estimate 10% +/- 3%) and prevalence of GAM is 10%. This means that you will have about 38 cases and 346 non-cases. That 38 is not a big number. It is made up for to some extent by the 346 but power will likley be well below 80% for detecting a relative risk of two or lower. the situation will be worse for less prevalent conditions. Once you start stratifying or using multivariate techniques to sort the confounders from the risk factors you will find that your have an insufficient sample size for a useful analysis. Anyway ... It seems to me that you are not collecting risk factors but a set of indicators. Indicators are not always (or often) risk factors. A more proper approach to investigating causality / associations would be to survey to see if there were a problem using sufficient identifying data to find cases later, work up cases found by (e.g.) taking histories, use this information to design a case-control study, perform a case-control study. I fear that you are trying to make a cross-sectional survey do something that it cannot do well. I know that some people do wide dataset cross-sectional surveys. This does not mean that it is right to do so in most situations. As far as I know SMART recommends a separate survey (nested) for mortality (if not you will have a survivor bias and probably a selection bias against older people) and a separate semi-quantitative food-security method based on SC-UK's "Household Economy Approach". Sticking in measles vaccination does little harm but not much good as you will only have a small sample size (you would use only the younger kids), reporting errors are common, and EPI is much more than a single measles vaccinaton. Morbidity data needs to be collected carefully using standard and stated case-definitions. A finding of an association between wasting and infection in any location is never going to be news. I can't stop you going out with a 20-page questionnaire. I can only advise againt it. Spare me an argument from consensus ... most people live in fear and abject poverty. The fact that this applies to most people doesn't make it a desirable state of affairs. The fact that you may have seem a lot of badly designed surveys don't mean that they were well designed. IYCF - more than 12 indicators? I'm surprised. All you need is a short feeding / food frequency questionnaire. It comes to thirteen variables (since you already have age). No problem doing (e.g. GAM and IYCF). No way can you have 50 variables and 12 indicators for one theme. That is just not useful. To answer your questions ... (1) Yes but why would you want to? I think you will not get meaningful results. (2) Difficult to answer. If you want results by district then take a decent sample in each district. Wide area estimates can be made by weighting during analysis. (3) You can't do this unless you, in effect, do different surveys in each district and you don't want to do that. If pushed to do this I would use a spatial sample, classify for small areas (ay less than 200 sq. km.) and map results using a traffic light system.

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