Theorising pathways of multi-dimensional scaling for population health using implementation and complexity science perspectives.

Scaling up is a complex process. As a multi-dimensional concept, it requires efforts to (1) increase population coverage (coverage), (2) expand or diversify what is included in the health service package (expansion), and/or (3) institutionalise a health innovation or new practice into health system services (institutionalisation). In this paper, we provide the theoretical basis for the model joining linear as well as complex pathways - stemming from implementation and complexity science - towards three-dimensional scaling. Our scale-up model positions expansion as the backbone for scale-up, and proposes multiple back-and-forth waves between institutionalisation and coverage. This allows an incrementalist approach, going step-by-step from one to the other scale-up dimension, as well as a multi-player complexity approach, emphasising the interactions between scale-up dimensions and actors involved, to achieving population health. By offering a dual incrementalist-complexity focus, we acknowledge that there is a starting point to scale-up, and thus path dependency, in addition to highly contextual cultural, historical, socio-political, and economic forces that underpin population health and any attempt at scaling up access and integration of health services, thereby pointing at their fragmented and incomplete nature. The visualisation and underlying hypotheses offer speculation on potential pathways for scale-up, which are key processes to understand and untangle in future research.