The is called sampling with “probability proportional to

The data used for this study were
obtained from food security and nutrition analysis unit (FSNAU). Food Security and Nutrition Unit
(FSNAU) is a unit in World Food Programme of United Nations (UNFAO), which was
set up in 1994 to provide evidence-based analysis of Somali food, nutrition and
livelihood security to enable both short-term emergency responses and long-
term strategic planning. Therefore, in partnership with UNICEF, FSNAU has been
conducting bi-annual seasonal nutrition assessment surveys since 2001. Our
study focuses on survey data ranging from 2007 – 2010Figure 1. Within this
period, FSNAUconducted cross-sectional nutrition assessment surveys twice a
year where information on falciparum
malaria parasitaemia was included upon request of United Nations Children’s
Emergency Fund (UNICEF).26,28. In each survey, a stratified multi-stage
cluster sampling design was adopted where the sampling frame of a selected
district was based on the four livelihood definitions (pastoral, agro-pastoral,
riverine and fishing) within which 30 communities and 30 households within each
village were selected using systematic  random sampling method and the urban population were clearly
defined and considered separately. Vulnerable groups that could not be
classified in any of the livelihood such as IDPs were surveyed separately. Respective
samples sizes (number of households and number of children) were calculated
using the Epiinfo/Ena 2008 software after considering the population size,
estimated prevalence and desired precision. A list of all villages and
population within each of the assessed livelihoods served as a sampling frame
and was used to construct cumulative population for the assessment area. Villages
were then randomly selected from these villages with the chance of any village
being selected being proportional to the size of its population. This is called
sampling with “probability proportional to population size (PPS)”. Selection
of households within the village was done using systematic random sampling,
preferably from a list of eligible names or a map of households. Where these
were not available, the number of households in the village was estimated from
the population figures (the total population divided by the mean household
size). Starting from a random household, every nth household was selected and
all eligible children (aged 6-59) in that household measured (Figure SI 1).
Retrospective mortality data was collected from all the households in each village
from each livelihood, including even those that did not have children aged 6-59
months. At the individual child level, age, gender, weight, height, mid-upper
arm circumference (MUAC), vitamin A supplementation in the last six months,
diarrhoea, acute respiratory infections (ARI) and febrile illness in the two
weeks before the survey, and Polio and Measles vaccination history were
collected. At the household level, information recorded included the household
size and age structure, gender of the household head, and access to different
types of foods in the last 24 hours. Data on falciparum malaria infection in children aged 5-59 months were
collected in sub-sets of villages at the request of UNICEF29–31. The data used in this study were
therefore a subset of the whole survey dataset with information on both the
childhood malnutrition and malaria.


We considered two outcome measurements
to describe the anthropometric indicators of malnutrition, low
weight-for-height (wasting) and low-MUAC, which detect different sets of children
as malnourished. Wasting is traditionally the main indicator in community
surveys. Although MUAC is a better predictor of mortality32, few studies have examined
associations between MUAC and specific pathogens.

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A child was defined as wasted when
s/he was below -2 Z scores for weight-for-height, according to World Health
Organization (WHO) 2006 standards33. A child with MUAC below 125mm was
classified as having low-MUAC. Malaria parasitaemia was determined using
Paracheck Pf™ (Orchid Biomedical Systems, Goa, India) in a subset of the sample
in every FSNAU surveys during this period33. A child was regarded as malaria infected
when s/he had a positive Paracheck Pf™ test result, regardless of any clinical


A detailed search were undertaken to
establish a set of spatial coordinates for each village in Somalia using the village
names in the data. The location of village was verified by using Google Earth
(Google, Seattle, USA) and other online databases to visually inspect whether
the coordinates matched evidence of human settlement. Those settlements for
which no reliable source of the coordinates was obtained were excluded from the