Study on if thanking donors has any impact on donating.  Answer = No (which is unexpected).  

Abstract

Calling to thank donors is a key fundraising strategy in the non-profit sector. Yet the effectiveness of these calls remains untested. We report on field experiments with public television stations and a national non-profit in which new donors were randomized to receive a thank-you call or not. The experiments involved about 600,000 donors and 500,000 thank-you calls over 6 years. We found a precisely estimated null effect of calls on subsequent giving. This result is in stark contrast to the incentivized forecasts of fundraising professionals and the general public, who anticipated that calls would increase donor retention by about 80%.

Samek, Anya and Longfield, Chuck, Do Thank-You Calls Increase Charitable Giving? Expert Forecasts and Field Experimental Evidence (April 13, 2019). Available at SSRN: https://ssrn.com/abstract=3371327