The Contextual Interference Effect

Quick Intro

The contextual interference effect is the finding that practice that is mixed, or interleaved, produces worse performance during practice and better retention and transfer later than practice that is repeated in blocks. The term was introduced by William Battig in the 1960s, and the key motor learning experiment was reported by John Shea and Robyn Morgan in 1979. Blocked and random practice have their own article, which covers how to apply it. This article covers the theory.

The Idea in Plain Terms

In the classic experiment, people learned three variations of an arm movement task. One group practiced each variation in a block, and another practiced them in random order. During practice the blocked group improved faster. When tested a week or ten days later, the random group performed better, particularly on a new variation.

Two explanations are offered, and they may both be true:

Elaboration. When tasks alternate, the learner compares and contrasts them, which builds richer memory about how the movements differ. This idea was developed by Shea and Zimny in 1983.
Forgetting and reconstruction. Switching tasks makes the learner clear the previous plan from short-term memory, so each attempt requires rebuilding the movement plan from scratch. The effort of rebuilding strengthens memory. This was proposed by Lee and Magill in 1983.

The effect fits a broader principle called desirable difficulty, from the memory researcher Robert Bjork: conditions that make practice harder in the moment can improve long-term learning.

The effect varies. It is stronger in laboratory tasks and in some sport and rehabilitation studies, and weaker or absent in other cases. Task complexity and learner skill seem to matter. Beginners with very complex tasks may do better with less interference, an idea captured in the challenge point framework of Guadagnoli and Lee in 2004, which holds that the best difficulty level depends on the skill of the learner and the nature of the task.

A Worked Example

A badminton learner practicing three serves:

Schedule	Order	Practice performance	Next-week performance
Blocked	10 short, 10 long, 10 flick	Good	Weaker
Random	Serves mixed unpredictably	Poorer	Stronger

The random schedule feels worse and teaches more.

What the Research Shows

The 1979 paper by Shea and Morgan in the Journal of Experimental Psychology: Human Learning and Memory established the effect in motor learning. A 2004 meta-analysis by Brady found a retention benefit for high interference practice, and found it was larger in laboratory tasks than in applied settings. Reviews of applied studies note that results in real sport skills are mixed and that effects are often smaller.

Studies suggest that learners often prefer blocked practice and believe it works better, which can lead coaches and learners to choose it for the wrong reasons.

Who It Suits

This concept helps:

Coaches designing practice sessions.
Learners who feel their progress is slow with mixed practice.
Students of motor learning.

How to Use It

Start with some blocked practice for first exposure.
Move toward mixed order as the pattern stabilizes.
Accept lower performance during practice.
Test retention later.
Match difficulty to the learner.
Mix skills that are related.

Common Misunderstandings

It is not that blocked practice is always wrong.

It is not a very large effect in every setting.

It is not the same as chaotic practice.

FAQ

Who proposed it? Battig introduced the term, and Shea and Morgan tested it in motor skills.

Why does it work? Probably through comparison and through repeated reconstruction.

Does it apply to kids? Studies show benefits for children in some tasks.

Does it apply to strength training? For skill learning, yes.

Where This Comes From

Shea JB and Morgan RL, 1979 (Journal of Experimental Psychology: Human Learning and Memory, 5, 179 to 187). Shea JB and Zimny ST, 1983, context effects in memory and learning movement information. Lee TD and Magill RA, 1983, the locus of contextual interference in motor-skill acquisition (Journal of Experimental Psychology: Learning, Memory, and Cognition, 9, 730 to 746). Brady F, 2004 (Perceptual and Motor Skills, 99, 116 to 126).