In a profession as complex as teaching, it seems obvious that we should provide educators with access to professional development that helps them keep improving their practice. But there’s also an obvious challenge, write James Zuccollo and Harry Fletcher-Wood.

Teachers’ time is limited, and so are school budgets: is professional development valuable enough to prioritise?

In the past, it’s been hard to answer this question.

However, the recent explosion in the number of robust experiments testing the effectiveness of professional development programmes allows researchers to draw wider and more confident conclusions about the difference CPD really makes.

Recently, researchers have begun to try to draw the findings of these experiments together to test the difference professional development makes.

The first such study to include English professional development evaluations is published today by EPI, in collaboration with Ambition Institute, and commissioned by the Wellcome Trust.

So how much difference does professional development for classroom teachers really make, and what can schools do to maximise the impact?

First and foremost, we found that professional development makes a significant difference to student attainment.

We conducted a meta-analysis, combining effect sizes (a statistic quantifying the impact the programme has had) taken from almost fifty CPD studies: the effect size of professional development was 0.09 with a 95 per cent confidence interval of 0.06 to 0.13.

A figure like that doesn’t tell us much on its own, but it becomes more meaningful if we compare it to other ways of improving student attainment.

For example, the effect of professional development appears to be equivalent to having a teacher in the classroom with over a decade’s experience.

Few interventions in schools have such a powerful impact, and the effect of professional development can be illustrated further if we compare it with two interventions with an even greater impact on student outcomes.

The first is wholesale in-school reform: the restructuring of a school, usually entailing radical change to the curriculum, staffing and processes. Professional development can have a powerful impact on students without the disruption, cost and dissatisfaction large scale upheaval causes.

The second is one-to-one tutoring for students. On average, it has an impact on student attainment three times larger than that of professional development. However, individual tutoring requires finding, training and paying these tutors. High quality professional development can improve outcomes for every student without breaking the bank.

The impact on students makes a compelling case for the importance of professional development; but we went further, to review the impact on teachers as well. We focused on teacher retention – a big concern, since one in five new teachers leave the profession within their first two years, while four in ten leave within five years and the cost to a school of a teacher moving on could exceed £10,000.

If teachers are being supported to improve, they should be happier, and more likely to stay – and the evidence we reviewed supported this hypothesis. Participation in professional development seems to increase retention for new and experienced teachers alike. In this context, we’re optimistic about the potential effects of the Early Career Framework and the additional support it offers.

Our study was also able to suggest ways to enhance the impact of professional development in schools. Making professional development easy and convenient for teachers is obviously important, and so is getting the right support for programmes from leaders: CPD that received sustained support from leaders was more likely to have a measurable impact. Alongside accessibility, continuity is important.

Professional development is no panacea and there are still big gaps in our knowledge of what kinds of professional development works best. But this report illustrates the role it can play, in both improving student learning and teacher retention.