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Estimating coverage in an exhaustive screening exercise

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Anonymous 730

Nutrition and Food Security Officer

Normal user

22 Feb 2018, 22:14

Dear all

We often conduct exhaustive MUAC screening and we calculate coverage for SAM and MAM using a simple formula which divides the number of children with SAM (or MAM) who are currently enrolled in the programme with the total number eligible.
I'm wondering how this way of calculating coverage is useful and also how ir compares with the point,period and single coverage estimator,particularly the single coverage estimator.This is because we intend to maintain the exhaustive screening as our primary method of obtaining coverage estimates.

Mark Myatt

Consultant Epideomiologist

Frequent user

23 Feb 2018, 09:44

Using:

    C.IN  = Current cases in the program
    C.OUT = Current cases not in the program 
    R.IN  = Recovering cases in the program
    R.OUT = Recovering cases not in the program

You have:

    C.IN / (C.IN + C.OUT)

This is the point coverage estimator.

Point coverage reflects the ability of a program to find and recruit cases. The point coverage estimator does not account for recovering cases and so does not directly reflect a program's ability to retain cases from admission to cure. This means that it may give a misleading (i.e. downwardly biased) picture of program performance. This is a particular problem with programs that recruit cases very soon after they meet program admission criteria and are likely to be treating large numbers of recovering cases. In these programs the point coverage estimator will "penalise" good performance.

Period coverage is:

    (C.IN + R.IN) / (C.IN + R.IN + C.OUT)

Period coverage is intended to reflect the ability of a program to find, recruit, and retain cases. The period coverage estimator directly reflects the program's ability to retain cases from admission to cure but tends to overestimate program performance because the denominator does not include recovering cases that are not in the program (R.OUT). The period coverage estimator also overestimates program performance when cases are retained in a program after meeting the criteria for being discharged as cured (e.g. to accommodate the absence of a supplementary feeding program). Period coverage is being replaced by single coverage.

A natural definition of program coverage would be:

    (C.IN + R.IN) / (C.IN + R.IN + C.OUT + R.OUT)

The problem with this estimator is that R.OUT (i.e. the number of recovering cases that are not in the program) is unknown and may be difficult to collect accurately. The single coverage estimator addresses this problem by estimating the number of recovering cases not in the program (R.OUT) using a simple mathematical model. Here is an FEX article describing single coverage.

What you have now is a restricted (and maybe biased) view of coverage. You can quite easily collect the additional data needed to use the period and single coverage estimators. I would also suggest that you use a simple barrier questionnaire as is using in SQUEAC stage III surveys and SLEAC surveys.

I hope this is of some use.

Anonymous 730

Nutrition and Food Security Officer

Normal user

26 Feb 2018, 21:24

This is super useful, thank you sir

Mark Myatt

Consultant Epideomiologist

Frequent user

28 Feb 2018, 09:51

Happy to help.

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