Risk Factor Selection Process
Third, to prioritize the risk factors for inclusion in PERCH,
we selected those with the highest population-attributable
fractions or with the strongest effects, while balancing pragmatic
concerns, such as feasibility, cost of data collection,
analytic plans, and comparability with existing data from
countries not represented in the PERCH study. An external
advisory group of pneumonia experts (the Pneumonia
Methods Working Group) [1] that was convened by the study
team assisted with further prioritization and suggested new
risk factors to assess.
At this stage, we gave particular consideration to newly
recognized potential risk factors. Vitamin D deficiency is a good
example. A case-control study in Ethiopia [19] suggested
a strong association (13-fold greater odds) between nutritional
rickets (caused by vitamin D deficiency) and pneumonia.
Likewise, an Indian study that measured serum 25(OH)D (25-
hydroxyvitamin D) concentrations suggested that subclinical
vitamin D deficiency is associated with pneumonia [14].
However, 2 more recent Canadian studies, both published in
2009, failed to find an association between serum 25(OH)D and
pneumonia [20, 21]. Although the role of vitamin D in pneumonia
risk remains unclear, vitamin D deficiency was not
identified as a priority by the Pneumonia Methods Working
Group. Due to the competing demand for serum needed to
assay vitamin D deficiency, it was not considered practicable to
evaluate the role of this vitamin in pneumonia.
Fourth, we defined the variables for the case report forms,
balancing questionnaire length against the need to be comprehensive
and to minimize the burden on study participants and
staff. Specific pragmatic and methodological considerations that
guided the variable selection and definition process are highlighted
in the sections that follow.