Omission vs. Commission Errors in ADHD Testing

If you’ve read a Continuous Performance Test report, two terms show up in almost every summary: omission errors and commission errors. They’re the two most-cited metrics a CPT produces, and for good reason — between them, they map onto the two symptom domains that define ADHD in the first place: inattention and impulsivity. But the terms themselves are easy to mix up, and the difference between them is worth being precise about, because they point clinicians toward different aspects of a patient’s presentation.

What an omission error actually is

During a CPT, a patient is shown a stream of stimuli and asked to respond to a specific target — pressing a key, for instance, whenever a particular letter or shape appears. An omission error happens when that target appears and the patient fails to respond to it at all. They simply miss it.

Omission errors are conventionally read as a marker of inattention. A patient who is missing targets is, in some sense, not tracking the task closely enough to notice them in the first place — their attention has drifted, whether from distraction, fatigue, boredom, or difficulty sustaining focus over the length of the test. A high omission-error rate, particularly one that climbs as the test goes on, is often read alongside real-world reports of a patient losing track of instructions, tuning out during long tasks, or needing things repeated.

What a commission error actually is

A commission error is close to the opposite: it happens when the patient responds to something that wasn’t the target at all. Instead of a missed response, it’s an extra one — a key press when nothing should have been pressed.

Where omission errors are read as inattention, commission errors are conventionally read as impulsivity, or as a failure of inhibitory control. The patient hasn’t failed to notice the stimulus; they’ve responded to it, or to something adjacent to it, without pausing to check whether a response was actually called for. In everyday terms, this maps onto the pattern of blurting out an answer before a question is finished, or acting before fully registering the situation.

Why the distinction matters clinically

Treating “errors” as a single undifferentiated number would flatten two genuinely different presentations into one score. A patient with a high omission rate and a low commission rate is telling a different story than a patient with the reverse pattern, even if their total error count is similar. The first pattern leans toward inattentive-type symptoms; the second leans toward hyperactive-impulsive-type symptoms. Neither pattern alone is diagnostic — CPT data is one input among several — but the split between the two error types gives a more specific signal than a combined score would, and it’s part of why CPT reports break them out separately rather than reporting one aggregate “error rate.”

The distinction also matters for tracking change over time. If a patient starts a stimulant and a follow-up test shows commission errors dropping sharply while omission errors stay flat, that’s a meaningfully different result than the reverse pattern, and it can inform how a clinician talks to a family about what, specifically, seems to be improving and what doesn’t yet.

Reading the two together, not in isolation

In practice, most patients show some combination of both error types rather than a pure pattern of one or the other, and that combination is worth reading alongside reaction-time data rather than on its own. A patient with a high commission rate and unusually fast reaction times is a fairly classic impulsive-response pattern: responding before fully processing the stimulus. A patient with a high omission rate and reaction times that grow slower and more variable across the test is a fairly classic sustained-attention pattern: losing engagement as the task goes on. These combinations, more than either error count alone, are what a CPT report is really trying to communicate.

It’s also worth remembering that a single test session is a snapshot. Fatigue, an unfamiliar testing environment, or simply an off day can shift either error rate without reflecting a real change in the patient’s underlying attention or impulse control. That’s one reason clinicians who use CPT data as part of an ongoing process — for instance, tracking response to medication changes — tend to look at the trend across sessions rather than treating any one test as the final word. A single high omission-error score is a data point; a rising trend across three sessions, alongside consistent real-world reports, is a much stronger signal.

The two error types can also shift independently across a titration process, which is part of why it’s worth tracking them separately rather than as a combined score. It’s entirely possible for a treatment change to bring commission errors down into a typical range while omission errors barely move, or vice versa — and a report that only surfaces a single aggregate number would hide exactly that kind of partial response, which is often clinically useful information in its own right.

The takeaway for interpreting a report

When you see omission and commission error counts on a CPT report, it’s worth resisting the urge to reduce them to a single “how bad is it” number. Read them as two separate signals — one pointing toward inattention, one toward impulsivity — and read both alongside reaction-time variability and the patient’s real-world history. Used that way, the split between the two error types is one of the more clinically useful pieces of information a CPT produces, precisely because it maps onto the two symptom domains you’re actually trying to evaluate.