Numbers that describe your list.
GROWTH RATE
New subscribers per month, against unsubscribes and removals.
A list shrinking steadily needs attention at the acquisition end.
CHURN
The proportion lost per period. Some is normal; a rise indicates a problem with frequency or content.
ENGAGEMENT PROPORTION
How many subscribers have opened or clicked anything recently.
A list where most subscribers never engage is smaller than it appears, and it damages deliverability.
BOUNCE RATE
Should be very low on an established list. A rise suggests list quality problems or an import of stale addresses.
COMPLAINT RATE
Should be near zero. Any rise needs immediate attention.
LIST AGE DISTRIBUTION
How many subscribers joined recently versus years ago.
Older subscribers engage less. A list that is mostly old and not growing is declining even if the total looks stable.
WHAT TO REVIEW QUARTERLY
Growth, engagement proportion, and complaint rate.
Those three describe whether the list is healthy.
THE HONEST NUMBER
Engaged subscribers, not total subscribers. That is the list you actually have.