Without an explicit reminder, time-based intentions are the ones most likely to be forgotten outright

What it is

With no explicit reminder in place, time-based intentions — do something at a given time, as opposed to when a given cue appears — are the type of prospective memory most prone to being forgotten outright. "Outright" is the operative word: this isn't about doing it late, it's about the window passing with the intention never resurfacing at all, until something unrelated jolts it back to mind much later.

Why it happens

Carrying out a time-based intention requires the person to periodically interrupt whatever they're doing and check "is it time yet" — a check that is entirely self-initiated, with no external signal ever stepping in to prompt it. Once the person is absorbed in something else, especially something that itself demands attention, the frequency of that self-check naturally drops, sometimes to zero — not out of carelessness, but because the task at hand is occupying the very attentional capacity that periodic checking would need. Once the gap between checks spans past the target window, the intention is missed entirely, and unlike an event-based intention, there's no later cue that can bring it back — with no arranged trigger in the environment, nothing downstream can reactivate it.

Studying it

The standard way to measure this is to have participants remember to perform an extra action at a specific time while engaged in a continuous, attention-demanding primary task, using clock-checking behavior — whether and how often participants voluntarily check the time — as a direct index of monitoring; a matched condition uses a functionally equivalent event-triggered task instead. The typical pattern is that as primary-task difficulty rises, monitoring frequency in the time-based condition drops and miss rate climbs, while the event-based condition's miss rate is far less sensitive to primary-task difficulty — this contrast is the most diagnostic evidence for classifying which type a given to-do belongs to.

Where it stops holding

If the consequences of missing a time-based intention are severe enough, or if the person naturally passes through a situation near the target time that indirectly prompts a check (glancing at the calendar out of habit near the end of the workday), the miss rate is markedly lower than for a purely isolated, unsupported time-based task — this account describes the worst case of "zero external support," not a guarantee that every time-based intention gets forgotten. Individual differences in self-monitoring ability and habits are also large, so a miss rate measured in one particular experiment shouldn't be treated as a universal figure.

Related

• Same group: A6.19.1 Prospective memory means remembering to act at a future time or occasion, distinct from recalling the past · A6.19.2 Event-based prospective memory relies on an external cue; time-based relies on internal monitoring · A6.19.4 Interrupting the current task drains prospective-memory resources, so more frequent interruption raises the odds of a missed to-do · A6.19.5 System reminders should bind to the triggering context, not just record to-do text
• Nearby: A6.06 The forgetting curve · A5.08 Interruption cost and task resumption
• Search terms: time-based prospective memory · forgetting intentions · self-initiated monitoring