By Lyndall Briggs, HACA, HMASCH
When everything goes perfectly, we may feel competent. Yet it is often the small,
noticed imperfection that tells us where growth is waiting.
We tend to speak about mistakes as though they sit on the opposite side of
competence. The skilled practitioner gets things right; the inexperienced practitioner
gets them wrong. It is an understandable distinction, particularly in professions
where judgement, ethics and the wellbeing of others matter. Yet it can also conceal
an important truth: once we become sufficiently accomplished to avoid every error,
uncertainty or awkward moment, we may also have become very good at remaining
within the boundaries of what we already know.
In a recent Big Think interview, author Daniel Coyle describes what he calls the rule
of 20 per cent. After visiting places that produced remarkable concentrations of
skilled musicians, athletes and performers, he noticed that their practice
environments had a different relationship with mistakes. They did not treat error as a
verdict on ability. They treated it as information.
The sweet spot
Coyle suggests that when people are succeeding approximately 80 per cent of the
time and making mistakes for the remaining 20 per cent, they may be working in a
productive learning zone. If everything is achieved easily, the task is probably too
familiar to require meaningful adaptation. If failure dominates, the task may be too
difficult, poorly structured or insufficiently supported. Between these two positions is
what Coyle calls the sweet spot: enough competence to remain oriented, and
enough difficulty to require genuine attention.
The figure should not be interpreted as a precise prescription. Nobody needs to
count mistakes until the required quota has been reached. Its value lies in the
principle beneath it. Learning requires us to reach slightly beyond our present
capacity, encounter a manageable difficulty, notice what happened and make an
adjustment. The mistake itself is not the teacher. What teaches us is the quality of
attention that follows it.
What deep practice looks like
Coyle illustrates this through the story of “Clarissa,” a young clarinet student
observed in research by Australian music psychologists Gary McPherson and James
Renwick. During one short practice period, Clarissa reportedly made progress
comparable with a much longer period of her ordinary rehearsal. Her practice did not
appear smooth or impressive. She played a small passage, noticed a wrong note, stopped,
looked closely at the music, hummed or silently rehearsed it, and tried again.
This is what Coyle calls deep practice. Rather than repeatedly performing an entire
piece and skating over its weaknesses, Clarissa remained close to the point of
difficulty. She slowed the process sufficiently to recognise the error and used it to
guide her next attempt. Her hesitations were not interruptions to learning; they were
the moments in which learning was taking place.
Later research offers an interesting echo of this idea. A 2019 mathematical modelling
study published in Nature Communications proposed an “85 per cent rule,” finding
that learning could be fastest at about 85 per cent accuracy under certain learning
conditions. That does not establish a universal human formula, and it should not be
stretched beyond the study’s assumptions. It does, however, reinforce the broader
observation that optimal learning often occurs when a task is neither effortless nor
overwhelming.
The supervisor’s learning edge
This has particular relevance for supervision. Experienced practitioners can become
highly skilled at bringing cases they understand, explaining interventions they can
justify and presenting work that already feels coherent. Such discussions can be
worthwhile, but they may not take either practitioner or supervisor very close to the
edge of learning.
The more fertile material often appears elsewhere: the moment the practitioner felt
uncertain; the intervention that did not quite land; the response noticed only
afterwards; the case that created an unexpected emotional reaction; or the familiar
method that suddenly seemed inadequate. These are not necessarily failures.
They are small openings through which something not yet understood becomes visible.
A useful supervisor does not manufacture difficulty or turn supervision into an
examination. The task is to create enough safety for uncertainty to be examined
without being converted into shame. Questions such as “Where did you begin to lose
direction?”, “What did you notice but not act upon?” and “What might you try
differently next time?” allow the practitioner to remain close to the learning edge.
The emphasis moves from proving competence to refining it.
The Art of Not Knowing
Coyle’s idea sits naturally beside the Art of Not Knowing. Not knowing does not
mean abandoning knowledge, standards or professional responsibility. It means
resisting the urge to close uncertainty too quickly. It permits the practitioner to remain
curious long enough for a more accurate understanding to emerge.
There is an important distinction here. The 20 per cent principle belongs in reflective
practice, skills rehearsal, education, role-play, case formulation and carefully
supported professional development. It is not permission to accept preventable errors
involving client safety, ethical responsibilities or serious harm. Good supervision creates
places where practitioners can explore the limits of their competence without asking clients
to carry the unnecessary risk of that exploration.
Perhaps the most valuable change is in how we interpret the experience of struggle.
If every imperfect moment is treated as evidence that we are inadequate, we will
naturally retreat towards what we can already do well. If it is treated as useful
information, we can become more precise, more thoughtful and more capable.
Growth rarely presents itself as a polished performance. More often it arrives as a
pause, a slight discomfort and the realisation that something could be done
differently.
The sweet spot is therefore not a celebration of getting things wrong. It is a
willingness to work where complete certainty has not yet arrived and to pay close
attention when it does not.
References
Coyle, D. (2026). “The rule of 20%: How to find the sweet spot for improved
performance.” Big Think. https://bigthink.com/series/full-interview/science-of-talent/
Coyle, D. (2009). The Talent Code: Greatness Isn’t Born. It’s Grown. Here’s How.
Bantam Books.
McPherson, G. E., & Renwick, J. M. (2001). A longitudinal study of self-regulation in
children’s musical practice. Music Education Research, 3(2), 169–186.
Wilson, R. C., Shenhav, A., Straccia, M., & Cohen, J. D. (2019). The eighty five
percent rule for optimal learning. Nature Communications, 10, 4646.
https://doi.org/10.1038/s41467-019-12552-4
Lyndall Briggs | sydneysupervisiontraining.com