Figure 1.

Schematic outlining the two stage Delphi study. In order to develop a core outcome set and definition/instrument set, Delphi methodology was used to gain consensus from expert groups. Two Delphis (stage 1 and stage 2) were carried out online over three rounds of questionnaires. The stage 1 Delphi focused on development of a core outcome set. The stage 2 Delphi focused on corresponding definition/instrument selection. PHE, Public Health England; SEF, standard evaluation framework; KPI, key performance indicator