The previously noted methodological decisions will result in many cases of severe pneumonia that will not be captured by PERCH. An inherent limitation of the focus on hospitalized cases is that we will miss cases who never come to hospital or who die at presentation before they can be enrolled. At the Kenya site, approximately two-thirds of children who die do so in the community [4 (link)], and only half of hospitalized children with severe or very severe pneumonia who died during their hospital stay participated in an etiology study [5 ]; thus, missed cases may represent those of greatest interest. This is an inherent paradox in hospital-based pneumonia etiology studies that must be accepted; however, to mitigate the consequences, we aim to collect postmortem specimens at 6 sites that may provide insights into the etiology of those who die at or near the time of presentation to hospital [6 ]. We will also miss the cases who seek care at nonstudy facilities or are not selected for enrollment because of sampling strategies. Although we cannot control the former situation, we will try to minimize the bias in the latter through sampling and analytic methods. The diversity of sites and the large sample size of PERCH will help ensure that we enroll a variety of cases, for which, using subgroup analyses, results can be generalized within these communities and to other similar communities.