Once you've sorted out tracking your ROI on various marketing activities the next phase is to 'nail' attribution. This is a great article that will help you do that.
The first click, last click, linear, and decay are all attribution models. They’re a set of rules that determines how your analytics tool applies credit for clicks, form completions, and conversions.
https://www.ruleranalytics.com/blog/click-attribution/attribution-modelling/
