Measuring the value of a loyalty programme

Our chain of twelve pharmacies runs a points-based loyalty card. About sixty percent of sales are linked to members, but we don't know whether the programme changes behaviour or simply rewards customers who would come anyway.

We are planning a test where some stores offer personalised offers based on purchase history, while others keep the standard points scheme. We also want to look at visit frequency before and after customers join. How have you measured the real impact of loyalty programmes beyond the number of members?