Key insight: LinkedIn and In-Profile sources collectively represent 25.3% of your total subscriber base but punch above their weight — they outperform the overall average on every quality metric. Engagement is +8.9 percentage points higher than average, and crucially, the unsubscribe rate is 9.2pp lower than the rest of the list. The in_profile source alone (Simon Harper's LinkedIn profile) accounts for 210 of those 309 subs and has the lowest unsubscribe rate of any major channel at 24.18%.