What should a marketer focus on to measure the effectiveness of email open rates?

The click-to-open ratio (CTOR) is a critical metric for measuring the effectiveness of email open rates because it reflects the proportion of recipients who not only opened the email but also engaged with its content by clicking on a link within it. This ratio provides valuable insight into how well the email's subject line and content resonated with those who opened it.

A high CTOR indicates that the email's content is compelling and relevant to the audience that chose to open it, showcasing the effectiveness of the email beyond just the initial open. In contrast, other metrics such as subscriber growth or click-through rates serve different purposes; subscriber growth focuses on the number of new individuals joining the mailing list, while click-through rates measure overall engagement with links in the email, not accounting for the initial engagement represented by the open. Compliance practices are essential for regulatory adherence but do not directly relate to measuring how effectively an email engages its audience after they have opened it. Thus, CTOR stands out as the most relevant measure for this specific scenario.