Here's how I'd go about it if I was in charge of the study:
Take a random sampling of accounts visited in a major US market and divide them into three distinct categories. Category A would receive a well written, polite and thoughtful thank you note following a sales call by a winery owner. Category B would receive a somewhat terse and bluntly honest note recounting the actual details of the visit. "Dear Joe, while I enjoyed the bagels in your state, frankly it really ticks me off that you claim you don't have a spot for our Fume Blanc... especially with summer right around the corner." And lastly, send no note or acknowledgement of the visit to the remaining portion of the accounts visited. Repeat this for several times throughout the year or throughout your career, whichever comes first. Then check the sales figures and accounts sold information provided by the distributor in that market to see if there was any impact as a result. In other words, did my efforts make any difference??
Personalization and authenticity in the wine business is becoming a thing of the past. I guess that's why I continue to write these silly little notes, thank you's and the like. But between you and me, I wonder if it makes all that much of a difference anymore.
Hmm... we will see when I get next month's accounts sold information.