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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.

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. http://www.en-net.org.uk/question/157.aspx 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. http://www.en-net.org.uk/question/212.aspx Cheers Kiross

Mark Myatt

Frequent user

20 Jan 2011, 09:54

Others have pointed out: http://www.en-net.org.uk/question/157.aspx and: http://www.en-net.org.uk/question/212.aspx 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.