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How to Maintain the Quality of CSAS Method

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Anonymous 310

Normal user

15 Jan 2010, 11:13

As we all know, coverage is an internationally recognized indicator of performance for selective feeding program (SFP) and CSAS is the current gold standard for assessing coverage of SFP. Since the information from the coverage survey help implementing organizations to strengthen programme performance and impact as well as for better future programming, I believe that the quality of the assessment procedure is the important facet of CSAS method. With this in mind, I have few concerns regarding CSAS procedures during field data collection

1. I came across with coverage reports that validation of case finding (capture-recapture study) was done by Active Case Finding vs. Central Location Screening, while the sensitivity of central screening is assumed to be low. This exercise will apparently gives a high estimate of sensitivity for case finding and subsequently over or under estimate the program coverage results. The capture-recapture technique guesses that house-to-house screening has higher sensitivity (90-100%) and similarly the field trial results in Malawi (Myatt et al, 2005) indicated that house-to-house method is more efficient than others. My question:- Is it acceptable to conduct validation study for case finding using central location screening while it yields less sensitivity than active case finding procedure?
2. Is it acceptable to take the typical sample size for CSAS (80 cases) in areas, where it is difficult to get updated malnutrition rates?
3. What is the minimum requirement of the area to be covered for the coverage survey in comparison with the total area of the district? For example, if the area covered by the survey (No. of quadrats X Area of each quadrat) is 75% of the total district area, is this design acceptable?
4. Is there any recommended way to decide the number of villages to be visited in each quadrat during case finding when the number of villages estimated in the sample size calculation is higher than the total number of quadrats? For example, if we get 30 quadrats during mapping, and we require visiting 35 villages, how can we decide the number of villages within each quadrat? Based on the number of villages in each quadrat; Population size; Physical content???
5. One of the attributes of CSAS as a replacement for the outmoded 30X30 method is that the active case finding is rapid and 6 communities can be sampled by one team in a day (http://www.fantaproject.org/ctc/myattPPT1.pdf). However, I believe that in most districts where SFP was established/operated, we expect to get more cases and/or covered (As rationale to establish SFP is high GAM & SAM). Therefore, if we request the team to cover 6 villages a day, we will compromise the exhaustiveness of finding cases (difficult to maintain the principle 'find all or nearly all') and likely to miss few cases/covered and as a result under or over estimate program coverage. Though, it helps to reduce cost of assessment, is it wise to plan for a team to cover 6 villages a day?

Hope to hear an expert opinion

Mark Myatt

Consultant Epideomiologist

Frequent user

15 Jan 2010, 13:38

To answer your questions in turn ...

(1) Yes. The technique come from biology / ecology to answer such questions as "How many beetles in this forest?" or "How many fish in the sea?". There is no expectation of exhaustivity. There are a number of assumptions ...

Closed population - not usually a problem for SAM but can be if central location screening attracts "outsiders".

Reliable identification and matching - Good training and proper collection of identifying data should ensure this.

Equal catchability - may be a problem if central location screening attracts "outsiders".

independence - Not usually a problem if different methods, staff, and informants are used.

What you should see here is that central location screening risks violating the closed population and equal catchabilty assumptions and may also, by its public nature, violate independence. The short answer is you can use central location screening but be careful. Note : You can identify and make rough corections for violations. See:

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

for more information.

(2) Sample size is not a straightforward issue. This has been discussed previously on this forum. The standard estimator for the number of cases (N) in the population sampled by a capture-recapture study is:

N = [((M - 1) * (C - 1)) / (R - 1)] - 1

Where:

M = number found by one method
C = number found by other method
R = number found by both methods

The estimator is unbiased if:

(M + C) > N

and:

R > 7

Provided you satisfy these constraints then you should be OK. You have to make guesses for N and the two sensitivities to do this. See:

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

for more information.

(3) You should attempt to cover as much of the area as possible. This will allow you to make a thorough map and avoid the bias that will come from excluding boundary areas.

(4) The usual approach is to decide on the spatial resolution of the survey. I do not usually go above a quadrat side length of 13 km (that gives 169 square kilometers). You can use more innovative sampling schemes and more complicated analysis to go bigger but retain resolution but I will not go into this here. You then draw the grid taking care not to introduce a bias by (e.g.) putting all sampling locations in valleys or by avoiding program area boundaries. You than sample as many communities as possible in a single day from each quadrat. I have seen quadrat sizes vary from 3-by-3 (Rwanda) and 13-by-13 (Niger). The number of communities per quadrat ranges from a mean of 3 to a mean of 5.

(5) Active case-finding usually only works well for SAM cases. I did some work on this during development of CSAS using WHM case-definitions and found something like 100% sensitivity for WHM < 70%, 70% sensitivity for WHM 70% - 74%, and 40% sensitivity for WHM 75% - 79% (I can find the exact figures if needed). This may improve a little for MUAC case-definitions but probably not enough to be useful. My advaice for moderate cases is to go house-to-house. MAM cases are more common than SAM cases so you don't need to sample as many communities. I am sure you will be able to think of alternatives if you need to do both SAM and MAM ... for example ... do active cases finding in 3 villages and door-to-door in a further 2 villages (first and third village) from each quadrat

I hope this is useful. let me know if I missed anything.

Mark Myatt

Consultant Epideomiologist

Frequent user

22 Sep 2015, 09:25

Did you ever do a capture-recapture study?

Could you share the results?

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