Year of data collection, different methodological choices and sampling strategies can certainly explain part of the variation [ 16].
Moreover, ORPHEA applied a robust methodological design and data collection tools to collect comparable information to estimate the cost of HTC, PMTCT, VMMC, and sex worker prevention services, the level of efficiency in the current delivery of these services, and the key determinants of efficiency – both within and across countries.
In this cohort profile, we aim to describe our study protocol, present its first results and ongoing data collection, and discuss the methodological issue of potential participation bias at study entry, which is one of the concerns for the external validity of cohort studies, for example, it is often assumed that the more healthy group is over-represented.
Differences in target populations, age range, data collection, diagnostic and classification criteria, methodological study design, and a lower reliability of retrospective studies, may account for such controversial results.
RM was involved in methodological development, data collection and input.
OS was involved in methodological development, data collection and input.
DW was involved in methodological development, data collection and input.
NW was involved in methodological development, data collection and input.
RC was involved in methodological development, data collection, data input and analysis.
JM-G was involved in methodological development, data collection, data input and statistical analysis.
