“…Therefore, an extensive body of academic literature examines how direct mailings could be developed even more successfully for NPOs. Besides the well-documented approaches of segmenting and targeting donors (Rupp, Kern, & Helmig, 2014;Sundermann, Boenigk, & Willems, 2017) and the improvement of the contents and design elements of direct mailings (Bekkers & Crutzen, 2007;Diamond & Iyer, 2007;Feld et al, 2013), another promising approach is to optimize the direct mailing frequency (Schweidel & Knox, 2013;van Diepen et al, 2009). Studies provide evidence that optimizing the direct mailing frequency over a defined time period affects the retention success in general (Elsner, Krafft, & Huchzermeier, 2004;Piersma & Jonker, 2004).…”