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Appropriate effect size in estimating sample size

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Anonymous 2356

Researcher

Normal user

19 Sep 2013, 20:16

What is the minimum rate of reduction in GAM in order to determine if an intervention is worth the investment? In other words, I am trying to get a sense of the appropriate effect size to use for calculating sample size in a control-treatment, nutrition outcome (acute malnutrition) impact evaluation.

Thank you,
Heather

Mark Myatt

Consultant Epideomiologist

Frequent user

20 Sep 2013, 13:18

We usually measure "worth the investment" in terms of cost per DALY averted. With wasting this is mostly about averting deaths. The problem comes with the definition of GAM. If this is based on W/H then the problem is that the functional outcome of (e.g.) having a WHZ of -2.76 varies from place to place so you may have difficulties calculating the number of deaths that you will avert with a given drop in GAM. If you use MUAC then you are on firmer ground with functional outcomes (since it tends not to vary much from place to place).

The basic approach will be to take survey data and, based on the prevalence and the distribution of your indicator (MUAC or WHZ), calculate an expected number of deaths (and disability ... death is total disability). Based on age and life tables you will then calculate the DALYs at the baseline and the DALYs at the endline. The difference between the two is your impact. Since you know the cost of the program you can then divide cost by the difference in DALYs to give a cost per DALY averted. For low-income counts anything less than about US$100 per DALY averted is considered "worth the investment".

You problem then becomes a "what-if" modelling problem (you could do this in a spreadsheet - I prefer a Monte Carlo simulation approach) to find the difference that will make it all worthwhile. The model can get complicated as you have to factor in coverage and because (if your program is responding to need) costs are not independent of prevalence.

Some general references:

Bobadilla, J. L., Cowley, P., Musgrove, P., & Saxenian, H. (1994). Design, content and financing of an essential national package of health services. Bulletin of the World Health Organization, 72(4), 653–662.

Anand, S., & Hanson, K. (1997). Disability-adjusted life years: a critical review. Journal of health economics, 16(6), 685–702.

Donev, D., Zaletel-Kragelj, L., Bjegovic, V., & Burazeri, G. (2010). MEASURING THE BURDEN OF DISEASE: DISABILITY ADJUSTED LIFE YEAR (DALY). WEB, 1–33.

Homedes, N. (1996). The Disability-adjusted Life Year (DALY) Definition, Measurement and Potential Use. HCD.

Rushby, J. F., & Hanson, K. (2001). Calculating and presenting disability adjusted life years (DALYs) in cost-effectiveness analysis. Health policy and planning, 16(3), 326–331.

Some recent CMAM related references:

Puett, C., Sadler, K., Alderman, H., Coates, J., Fiedler, J. L., & Myatt, M. (2012). Cost-effectiveness of the community-based management of severe acute malnutrition by community health workers in southern Bangladesh. Health policy and planning. doi:10.1093/heapol/czs070

Bachmann, M. O. (2009). Cost effectiveness of community-based therapeutic care for children with severe acute malnutrition in Zambia: decision tree model. Cost effectiveness and resource allocation : C/E, 7, 2. doi:10.1186/1478-7547-7-2

I hope this is of some help.

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