As a Program Owner I say: joins. Sales. NOT rebills. Huh? Why?
Because Members Areas and rebills are what I can control most - making the MAs the best they can be, providing excellent and abundant content and updates, etc - but TRAFFIC is what I desire, crave and spend most of my time generating. So when rebills are strong but sales are weak that is, to me, BAD.
Example: You get 50 transactions on Monday and 50 transactions on Tuesday.
Monday: 40 rebills and 10 sales
Tuesday: 40 sales and 10 rebills
So which day is the better day? To ME it's Tuesday and Monday is a disaster! Why? Because the traffic generated on Monday should have produced 40 sales but did not.
So if sales are down that is bad. If rebills are down that is bad, too, but theoretically i should have more control on how to boost rebills while I do not always have control over traffic, conversions, joins, Visa, scrubs, etc.
For Affiliates I would imagine it would be rebills.
