Every Continuous Performance Test reports two error types: omissions (failing to respond to target stimuli) and commissions (responding to non-target stimuli). They measure different cognitive constructs, and the ratio between them carries clinical information beyond either metric alone.

Omission errors — vigilance and target detection

An omission error is the patient seeing a target stimulus and failing to press the button. The cognitive process being indexed is sustained attention or vigilance — the ability to maintain attentional focus across a continuous task long enough to detect each target.

Causes of elevated omission rates:

Inattention / mind-wandering during the task.
Lapses in vigilance — particularly common in long CPT paradigms.
Drowsiness / sleep deprivation (significant confound).
Sub-effortful task engagement (rule out before attributing to ADHD).
In some cases, slow processing speed coupled with strict response windows.

Omission errors are most strongly associated with the inattentive presentation of ADHD. Studies of ADHD-Inattentive type generally show elevated omissions with relatively preserved response inhibition.

Commission errors — response inhibition

A commission error is responding to a non-target — pressing the button when you shouldn't have. The cognitive process is response inhibition — withholding a prepotent response when the stimulus indicates not to respond.

Causes of elevated commission rates:

Impulsivity / poor inhibitory control.
Strategy errors — patient prioritizing speed over accuracy.
Difficulty distinguishing target from non-target (perceptual confound).
In go/no-go paradigms with high target probability, rate-of-non-target relative to total responses is higher and small response-control failures produce higher absolute commission counts.

Commission errors are most strongly associated with the hyperactive/impulsive presentation of ADHD. ADHD-Combined type typically shows elevations in both metrics.

The omission-to-commission ratio

The ratio carries information that absolute counts alone don't:

High omissions + low commissions: classic inattentive pattern. Strategy is conservative — patient under-responds when uncertain.
Low omissions + high commissions: impulsive pattern. Strategy is liberal — patient responds quickly with reduced inhibition.
High omissions + high commissions: combined inattention + impulsivity, or sub-effortful engagement. Critical to rule out the latter.
Low omissions + low commissions: typical performance, or possibly excessive caution producing slow but accurate responding.

This ratio is sometimes formalized via signal detection theory metrics (d-prime, response bias) reported by some CPT tools (notably TOVA and the Conners CPT-3).

Common interpretation pitfalls

Treating elevated commissions as universally diagnostic of impulsivity. Strategy and motivation matter. A patient told to respond as quickly as possible will produce more commissions; one told to be careful will produce more omissions.
Ignoring engagement. Sub-effortful engagement produces high omissions that mimic vigilance failure. Look for patterns: random-looking responses, perfect performance early then deterioration, or RT outliers.
Forgetting medication state. Medication-on vs. medication-off CPTs produce very different error patterns. Both can be useful, but interpretation depends on which.
Single-administration over-interpretation. CPT performance is sensitive to time-of-day, sleep, caffeine, and fatigue. Treat single results as estimates, not point measurements.

What error patterns inform clinically

Beyond DSM-5 presentation typing:

Treatment selection. Patients with predominant impulsivity may respond differently to alpha-2 agonists than to stimulants alone, particularly when comorbid tics or oppositionality coexist.
Titration target. Treatment goal can be framed in terms of normalizing the elevated metric — useful for patient communication ("we're trying to bring your commission rate from the 95th percentile down toward 70th").
Response monitoring. Within-subject change pre-/post-medication is more meaningful than cross-sectional norm comparison. A 30% reduction in omissions on medication is a clinically meaningful signal independent of where the patient sits on population norms.

Use the score interpretation calculator to ballpark percentile ranges, or compare specific tools in the comparison page.