Silently killing your panelists one email at a time: The true cost of email solicitations

Marketing firms routinely interact with their panelists via email. While sending an invitation to respond to a survey may seem virtually costless, over-solicitation could lead to panelists unsubscribing or ignoring future emails. Since online panels are a crucial resource for a marketing research firm, such attrition is a major issue. We account for the unobserved cost of solicitations in a joint model of response and attrition propensities. Using a data set of more than 150,000 email solicitations sent over three years, we demonstrate that additional solicitations not only temporarily decrease the likelihood of future participation but also increase the attrition rate, likely due to wearout. The model where solicitations “kill” panelists outperforms out of sample a benchmark model that assumes dropout is caused by the passage of time instead. Since the impact of solicitations is both transient (on the response model) and permanent (on the dropout process), managers should wait for the temporary impact to dissipate before risking to “kill” their panelists with another solicitation. We illustrate the economic importance of this finding using a differential evolution method that optimizes the firm’s solicitation strategy under different scenarios and show a 30.7% improvement. In the long term, a greedy strategy (targeting the best-responding panelists) performs worse than a random policy.