The cost of interruption lands on task resumption

What it is

The cost of a notification is not the seconds spent reading it but what it interrupted: after the notification ends, the interrupted task must be rebuilt — recalling where one was, restoring context, re-entering the state. The bulk of interruption cost lands on task resumption (its existence is established in interruption research and assumed here), and notification metrics book none of it — a textbook externality.

Why it happens

The externality's structure is that benefit and cost enter different ledgers. The benefit — opens, return visits, conversions — is booked precisely on the sender's dashboard, attributable and optimizable; the resumption cost is scattered across every task the user had interrupted, appearing on no notification-related board. So "send more" looks free in every local decision: not because there is no cost, but because the cost is not on the decision-maker's table. Private optimization against private accounts while social costs go unbooked — the old structure from environmental economics, replayed exactly in notifications. Frequency experiments therefore always come out "significant": the measurement window can only see the benefit side.

Where it stops holding

Attributability of the cost varies with the task: deep creative work is most expensive, mechanical tasks cheap, and averages mask the distribution. Expected, user-scheduled breaks (checking while waiting) carry near-zero resumption cost — the cost comes from unexpected interruption, not from notifications existing. Internalizing the cost does not mean "never send": genuinely valuable notifications stay net-positive once costs are booked; internalization only lets the sending decision see the full account.

Applying it

Book interruption cost into notification decisions: generate an explicit cost line per category — average resumption time of interrupted tasks times affected users — displayed beside open-rate benefit on the same report, weighted by the task mix (shallow/deep share) rather than one mean. Tier notifications — may interrupt now, summary into a tray, silent accumulation — with most categories non-immediate by default and users unlocking immediacy for a few. Verification: for the same batch, compare immediate push against digest batching and read the sender-side difference — that difference is the externality's share of the profit, and it belongs in decision materials as a cost.

Related

• Same group: P3.06.1 Notifications can manufacture sessions the user never initiated · P3.06.2 Recall unrelated to user goals is attention taking · P3.06.3 Frequency judged by user value, not retention · P3.06.5 Badges are content-free recall signals · P3.06.6 Permission granularity determines whether users can refuse in part · P3.06.7 Timing more than frequency decides whether notifications read as harassment
• Adjacent: A5.08 Interruption and resumption cost · P3.03.1 Optimize for the user's long-term goals
• Search terms: interruption cost · externality · batched notifications · task resumption