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Calculating beneficiary numbers of SAM/MAM using prevalence

This question was posted the Assessment and Surveillance forum area and has 3 replies.

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Ali Maclaine

Normal user

19 Jan 2011, 18:41

Hi, I am trying to work out estimated beneficiary numbers using prevalence and incidence for SAM and MAM. For SAM as I understand it you take the total population of your area e.g. 450,000, then take estimate of under five population 20%. We have the prevalence 3.1% that gives 2790 (450000*20%*3.1 = 2790). But we know that the actual number of children to treat every year is two to three times higher than prevalence. Is it 2 times higher or 3 times higher? Assuming it is 3 then I have been told different methods of calculating it, is it Using example above 2790+ (2790*3) = 11160 OR again using example above 2790*3 = 8370 ? Then how do you do this for MAM? THANKS! Confused of London!

Rogers Wanyama

Emergency Nutrition Specialist

Normal user

20 Jan 2011, 05:17

Hi Annonymous 169 A similar question was posted in 2009. Log to the link below. Hope it helps Thanks Rogers

Anonymous 81

Public Health Nutritionist

Normal user

20 Jan 2011, 07:53

Dear Annonymous 169 To add on Rogers. you can also find similar question with reply from the following link. Cheers Kiross

Mark Myatt

Epidemiologist at Brixton Health

Frequent user

20 Jan 2011, 09:54

Others have pointed out: and: One approach is to use the formula: EXPECTED = POP * U5% * EP% * CFI where: POP = Total population U5% = Percentage of total population age 6-59 months EP% = Estimated prevalence CFI = Correction factor to estimate incidence from prevalence The value of CFI is uncertain (see the links above). A CFI of 2.0 (in this formula) is broadly in-line with published estimates. This level of CFI is used for SAM. I do not think we have a good idea of the value of CFI for MAM (so I would use 2.0). The problem with: EXPECTED = POP * U5% * EP% * CFI is that it takes a "fairy tale" view of programming in the sense that it assumes a coverage proportion of 100%. I have looked at coverage of TFC, OTP (in both CTC and CMAM guises) and SFP and have never seen coverage above 89%. Here are some rules of thumb for different program types: TFC : Typical range 0.5 - 5% (maximum seen is c. 30%) OTP : Typical range 20% - 80% (minimum 8%, maximum 89%) SFP : Typical range 5% - 20% (limited data available) We have to face it ... most programs achieve coverage below SPHERE minimum standards. The point is that we need to account for coverage in the formula: EXPECTED = POP * U5% * EP% * CFI * COVERAGE Using your data and assuming the program will hit the SPHERE minimum of 50% we have: EXPECTED = POP * U5% * EP% * CFI * COVERAGE EXPECTED = 450000 * 20% * (3.1 / 100) * 2.0 * (50 / 100) EXPECTED = 2790 Another approach is to use the formula: EXPECTED = POP * U5% * EP% + (POP * U5% * EP% * CFI) That is prevalent cases + incidence cases. A value of 1.6 is used for CFI (this is a published estimate). This level of CFI is used for SAM. I do not think we have a good idea of the value of CFI for MAM (so I would use 1.6). This formula also fails to account for coverage. A better formula is: EXPECTED = POP * U5% * EP% * IC + (POP * U5% * EP% * CFI * AC) where: IC : Initial phase coverage (often low) AC : Achieved coverage (i.e. after the first few months) It is sensible to use: IC = AC / 2 as the average between starting at zero and achieving 50% some time later. Using your data and: AC = 50% and: IC = 50% / 2 = 25% we get: EXPECTED = 450000 * 0.2 * 0.031 * 0.25 + 450000 * 0.2 * 0.031 * 1.6 * 0.5 EXPECTED = 2930 The two methods give similar answers. You have to be aware that there are big sources of error in both of these approaches: POP : Subject to secular change but also displacement and migration U5% : Subject to secular change &c. and public health shocks CFI : An informed guess base on limited data EP% : For SAM this will be very imprecise (e.g. 1.15%, 95% CI = 0.38%; 2.57%). And COVERAGE is not known. We have to be realistic about what we will achieve. I'm sure that the agencies that "achieve" 8% coverage started out thinking they would get 80% coverage. BEWARE : You need to use EP% for your program admission criteria. If you use MUAC then EP% is for the MUAC case-definition not the W/H case-definition. So ... a short answer ... there is no really correct way. There are different ways of getting an informed guess which, given the same assumptions, give roughly the same answers. I hope this helps.

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