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New Sampling Methods for Surveying Nutritional Status. Megan Deitchler June 28, 2007. Presentation Outline. Traditional Approach to Assess Nutritional Status Work to Date on New Sampling Methods Comparison of New Sampling Methods with Traditional Approach. Data Priorities in Emergencies.
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New Sampling Methods for Surveying Nutritional Status Megan Deitchler June 28, 2007
Presentation Outline • Traditional Approach to Assess Nutritional Status • Work to Date on New Sampling Methods • Comparison of New Sampling Methods with Traditional Approach
Data Priorities in Emergencies • Statistically representative data on the status of a situation are usually necessary before resources can be directed to the area • Reliable data are needed rapidly – to allow agencies to make appropriate decisions about interventions and the level and type of aid to provide • Often times recurrent assessments are mandated (quarterly, e.g.) – either by national government or donor agencies • Data on GAM among children 6-59 mo is a key indicator used to measure the severity of a situation
The Traditional Approach 30x30 Cluster Survey # Clusters: 30 # Observations per Cluster: 30 Total Sample Size: 900 How to Collect 30x30 Cluster Data: • Obtain a complete sample frame • Conduct Population Proportionate to Size (PPS) for cluster selection • Establish the random start point within a cluster • Administer questionnaire based on demographics of HH • Sample next nearest HH until data are collected for 30 HHs and 30 Children in the Cluster REPEAT STEPS 3-5 in 29 OTHER CLUSTERS SELECTED BY PPS
The Traditional Approach Challenge Conventional 30x30 cluster surveys are time and resource intensive – this poses a challenge, particularly in emergency situations, when rapid and appropriate humanitarian response is essential for effective targeting of scarce resources to communities most in need.
New Sampling Methods ObjectiveTo develop a statistically representative assessment method allowing for rapid collection and analysis of data. Result 1. 33x6 LQAS Design 2. 67x3 LQAS Design
Comparison of Traditional and New Sampling Methods 30x30 Design, 33x6 Design, 67x3 Design Accuracy Precision for child- and HH-level indicators using data from a field validation in West Darfur, Sudan
Vocabulary • Intra Cluster Correlation A measure of how similar the responses are within clusters for an outcome of interest. The intra cluster correlation coefficient is often referred to as rho, and is designated by “p”.
Why Do The LQAS Designs Work? • Inherent to cluster sampling is the possibility for intra cluster correlation. • The greater the intra cluster correlation, the more data (clusters) must be collected to precisely capture the variability that exists across the entire assessment area.
Vocabulary • Design Effect A measure of the extent to which a cluster design has diminished ability to precisely measure an indicator, as compared to a simple random sample of the same size. DE=1 + p (m-1), where m=# obs per cluster
Underlying Principles of LQAS • LQAS analysis methods can be used to assess binary outcomes • Allows for hypothesis testing of an indicator against threshold prevalence levels • Statistical principles of the method are based on cumulative probabilities of binomials • Decision rules are used to judge whether the threshold prevalence of an indicator has been reached • Requires observations be independently and randomly selected (SRS)
Why Call the Methods LQAS Designs? • The 33x6 and 67x3 designs allow for LQAS hypothesis tests of GAM prevalence because the designs approximate a SRS for assessment of GAM • 10% and 15% prevalence levels (i.e. upper thresholds) of primary interest, 20% prevalence of secondary interest • Useful if 95% CI overlaps with a threshold level necessary for decision making about response
Summary of Main Points • 33x6 and 67x3 designs provide accurate results for child- and HH-level indicators • 33x6 and 67x3 designs provide reasonably precise results for child-level indicators, with a CI only slightly wider than that of a 30x30 design for most indicators. • Note: the 67x3 design provides more narrow CIs than the 30x30 design for child-level indicators with high intra-cluster correlation (eg. VAC suppl., diarrhea, malaria). • 33x6 and 67x3 designs provide reasonably precise results for HH-level indicators (notable exception is mortality). • Note: the 67x3 design provides more narrow CIs than the 30x30 design for all HH-level indicators tested (exception is mortality).
Summary of Main Points • 33x6 and 67x3 designs allow for LQAS hypothesis testing of GAM threshold prevalence levels (using cumulative binomial probabilities) • 33x6 and 67x3 designs offer substantial benefit over 30x30 cluster design in terms of sample size, time, and cost required for data collection • 33x6 and 67x3 design are statistically appropriate alternatives for purpose of obtaining rapid, reliable assessment data in food insecure areas
For More Information FANTA website http://www.fantaproject.org Megan Deitchler mdeitchl@aed.org
Acknowledgments Financial support for this study was provided to the Academy for Educational Development (AED), Food and Nutrition Technical Assistance (FANTA) Project by the Office of Foreign Disaster Assistance (OFDA) of the Bureau for Democracy, Conflict and Humanitarian Assistance, and the Office of Health, Infectious Diseases and Nutrition of the Bureau for Global Health at the US Agency for International Development (USAID), under terms of Cooperative Agreement No HRN-A-00-98-00046-00. The opinions expressed herein are those of the author and do not necessarily reflect the views of USAID.
