Error vs Mistake vs Slip

In ELT and SLA, not all non-target forms are the same. The three-way distinction between errors, mistakes, and slips, rooted in Corder (1967) and refined by Edge (1989), has practical consequences for how teachers respond to learner language.

The Three Categories

Category	Cause	Systematic?	Self-correctable?	Example
Slip	Tiredness, nervousness, distraction: a performance lapse	No	Yes, immediately when pointed out	A learner who knows the past tense says I go yesterday in a fast-paced conversation, then self-corrects: went
Mistake	The learner has been taught the form and can sometimes produce it correctly, but has not fully internalized it	Semi-systematic	Yes, with prompting or time to reflect	A learner uses much and many correctly in exercises but confuses them in free writing
Error	A gap in competence: the learner does not yet know the correct form	Systematic	No, the learner cannot correct it even when asked	A Vietnamese learner consistently drops articles because the L1 has no article system

Corder's Original Distinction (1967)

Corder drew the line between competence and performance, following Chomsky:

• Errors of competence are systematic. They reflect gaps in the learner's Interlanguage: rules that have not yet been acquired. The learner produces the wrong form consistently because they do not know the right one.
• Errors of performance (mistakes/slips) are unsystematic. The learner has the knowledge but fails to access it due to processing pressure, fatigue, or inattention.

Corder's key diagnostic: Can the learner correct it when asked? If yes, it is a mistake or slip. If no, it is an error.

Edge's Practical Refinement (1989)

Julian Edge, in Mistakes and Correction (1989), offered a more teacher-friendly three-part classification:

Edge's term	What it means	Teacher response
Slip	The learner knows the form and can self-correct	Indicate the problem; let the learner fix it
Error	The learner has been taught the form but gets it wrong; the form is in their syllabus but not yet secure	Reteach, provide models, give practice
Attempt	The learner has never been taught the form and is trying to express something beyond their current level	Note it for future teaching; do not penalize

Edge's "attempt" category is particularly useful because it reframes certain non-target forms positively: the learner is taking risks and trying to communicate beyond their current ability, which is exactly what communicative approaches encourage.

Why the Distinction Matters

For correction decisions

The distinction directly affects whether and how a teacher should respond:

• Slips often need no correction at all; the learner will self-correct if given a moment. Over-correcting slips can damage rapport and increase anxiety (raising the Affective Filter).
• Errors require teaching intervention, including Corrective Feedback, focused practice, or reteaching, because the learner cannot fix what they do not know.
• Mistakes (Edge's middle category) benefit from prompting strategies: self-correction cues, recasts, or elicitation that helps the learner access knowledge they already have.

For assessment

In Formative Assessment, the distinction helps teachers decide which non-target forms to flag. Slips under exam pressure are expected and should be weighed differently from systematic errors that reveal gaps in competence.

For research

Error Analysis only studies errors (systematic gaps), not slips or mistakes. Mixing the categories produces unreliable data; a researcher who counts slips as errors will overestimate the learner's difficulties.

The Diagnostic Challenge

In practice, the line between error and mistake is often blurry. A learner who produces a correct form in a controlled exercise but an incorrect form in free speech may have:

• Partially acquired the form: correct in controlled practice but not yet automatized for fluent production
• Backsliding: a form that appeared acquired resurfaces as an error under communicative pressure (see Fossilization)
• A processing limitation: the correct form is known but requires too much cognitive effort to deploy alongside other demands

The only reliable diagnostic is repeated observation across contexts. A form that is consistently wrong across tasks and conditions is likely an error; one that fluctuates is a mistake.

References

• Corder, S.P. (1967). The significance of learners' errors. IRAL, 5, 161–170.
• Corder, S.P. (1981). Error Analysis and Interlanguage. Oxford University Press.
• Edge, J. (1989). Mistakes and Correction. Longman.
• James, C. (1998). Errors in Language Learning and Use: Exploring Error Analysis. Longman.
• Brown, H.D. (2007). Principles of Language Learning and Teaching (5th ed.). Pearson.