Three numbers tell the story, and none of them is the star rating on its own. The number of new reviews in the period, because that shows the ask is working. The share answered and how quickly, because that is the thing we are directly responsible for. And enquiries — calls, forms, messages — because that is what the business actually needs, and the reason the reviews matter.
What we can measure honestly: how many reviews arrived, on which sites, what they said, how fast each was answered, how the rating moved, and how enquiries moved in the same period. What nobody can measure honestly is the counterfactual. We can show the rating rose and calls rose; we cannot prove one caused the other, and any agency presenting that as proven is overstating what the data supports.
One warning about the number everyone watches. A rating that moves from strong to perfect is not necessarily good news — a wall of identical five-star reviews with no detail reads as manufactured, to prospects and to Google. Real reviews are uneven, and a business with real reviews and calm replies to the bad ones is more trusted than a business with none.
Reviews do their work over months. If you need the phone ringing sooner, paid search can put the number in front of people from the first week, and a good rating makes every ad cheaper to run.