Press release: List size is one of the strongest predictors of email response rates, including both open and click-through responses, according to a new study by ExactTarget.

There appears to be an inverse relationship between list size and email responsiveness. According to the study, open and click-through rates both decrease steadily as list size increases.

“This phenomenon is one of the strongest cases for audience segmentation,” according to Morgan Stewart, director of strategic services at ExactTarget and author of the study. “The smaller the targeted audience, the better organizations can aim their message directly to their subscribers in their email communications. This is a sure way to increase response to email communications.”

The study also shows that organizations with a B2B focus experienced higher open and click-through rates than their B2C counterparts. This is partially explained by the average size of lists to which these organizations send email. Fifty-two percent of B2B email campaigns were sent to lists of less than 1,000 subscribers, and only 5.6 percent of B2B campaigns were sent to lists of more than 10,000 subscribers. Compare that with 41 percent of B2C campaigns sent to lists of more than 10,000 subscribers.

The downward trend in open rates by list size stabilizes at between 15 to 20 percent average open rate when reaching 400,000 to 500,000 subscribers. At this point, the audience appears to be of significant size so that adding incremental names to the list does not have an increased negative affect on open rates.

The study shows that the downward trend in click-through rates is less predictable than open rates. While this trend still exists, there is more variability in average click-through rates at higher volumes. Estimation models based solely on email volume are not particularly accurate, indicating that email creative and content play a crucial role.

“The larger the list, the more important it is to have compelling creative and offers in order to generate click-through rates in your campaign,” said Stewart. “With larger lists, it is much more difficult to drive significantly higher open rates than normal. This is due to both list fatigue inherent in larger lists and the reality that it is difficult to convey compelling and relevant messages to a large audience through the subject line.”

The ExactTarget 2005 Response Rate Study summarizes overall open, click-through and unsubscribe rates and provides additional analyses based on day of week for sending email while examining list size and target audience.