Response to reviewers' comments: Reviewer one: TITLE The “Part 1” in the title, while perhaps intriguing, calls for some explanation of future directions.
Also, the difference(s) between FFM and LM definitions are nuanced and complex and should either be defined, or alternate text should be used here. It seems that the authors are simply trying to point out that use of the 4C model is not common and that other methods are normally used. Best to clarify this statement. INTRODUCTION P2 L61: FFM is obtained via the 4C model – it is not a surrogate.