How do I measure the impact of frequency on engagement and unsubscribes?

You're testing whether sending more or less often helps your business. That's smart. But most people measure the wrong metrics and end up with misleading conclusions.

Here's what actually matters. You need to track per-email metrics and cumulative metrics at the same time.

Per-email metrics show degradation: Open rate per email (does it drop as frequency increases?). Click rate per email. Revenue per email. These tell you if each individual message gets weaker.

Cumulative metrics show the payoff: Total opens across your test period. Total clicks and conversions. Total revenue generated. This reveals whether the extra volume makes up for lower individual performance.

Attrition metrics matter most: Unsubscribe rate per email. Total unsubscribes over the test period. Complaint rate. List decay rate. (Of course, easier said than done, not all ESPs report these equally.) These show the long-term cost.

The real question is this: higher frequency might spike your short-term revenue, but does it kill your list faster? Model the long-term economics. A test that runs 2-4 weeks usually isn't long enough to see the real attrition pattern. Aim for 6-8 weeks if you can.

Start by picking your test groups (high frequency vs. control) and deciding: what's your primary win metric? Revenue per subscriber? Total revenue? List health? That choice changes which metrics matter most. Once you run the test, build a simple spreadsheet comparing all these side-by-side so the tradeoff becomes obvious.