Once upon a time, not so long ago, teachers were judged by means of a single digit that graded an hour’s performance, observed through the narrow lens of one lesson, with one class, and with the biased eyes of one observer.

Tell your children that and they won’t believe you. “But that’s ridiculous,” they’ll protest. “Surely no one thought that made any sense?” And the answer, of course, is the secret hidden in plain sight, the elephant in the room: it didn’t.

If your college is still grading lesson observations and using this score as a proxy for quality assurance and performance management, then you have my deepest sympathies. I’m confident, though, that the tide has finally turned, albeit slightly later in FE than was the case in schools.

I’m not quite so confident, however, that most colleges have managed to find a workable alternative. When it comes to performance management, my philosophy is simple: it’s no one’s vocation to fail.

In other words, no one wakes up in the morning determined to do the worst job they possibly can. No one looks themselves up and down in the mirror and vows to fail as many students as they can before nightfall. But despite the best intentions, sometimes some people don’t perform as well as we’d like.

When teachers underperform, they need to be given time and support - including training - in order to improve. And many of them will. But those who don’t need to leave the profession. Retaining people who cannot perform their duties serves no one well, least of all our students.

Accountability, when managed fairly and accurately, is a good thing. It ensures the best people do the best jobs.

Arguing against formal, graded lesson observations is not, therefore, akin to arguing against the need for accountability. But lesson observations do not accurately or reliably measure the quality of teaching.

As a result of high-stakes lesson observations, teachers tend to do one of two things:

1. They over-plan, over-teach and proffer showcase lessons that bear absolutely no relation to their everyday practice.

2. They become stressed by the experience of being watched, and so underperform.

But even if a teacher is brave enough to teach a “normal” lesson, the very presence of an observer in the room inevitably alters the classroom dynamics. This is called the Hawthorne Effect.

So what’s the answer?

Let me be clear: I’m not suggesting we stop observing lessons altogether. I think walking into lessons to see what’s happening is important. By observing the classroom environment, we can see the rapport the teacher has established and how well the teacher manages behaviour and utilises resources.

But to accurately judge the quality of teaching, we need to triangulate what we see and hear in classrooms with other sources of information, not least our much-maligned but vital professional judgment. We should measure the quality of teaching in a holistic way.

I favour a “balanced scorecard” approach - a means of aggregating a range of data. This can be drawn from in-year student progress, student outcomes (including value-added scores), retention, student voice surveys, a teacher’s contribution to their own and others’ professional development, evidence against the professional standards and so on. The wider the net is cast, the more accurate, fair and holistic the picture of performance will be.

Another advantage of a scorecard is that it can focus attentions where they’re needed most and help drive positive behaviours.

A scorecard places data centre-stage and puts it in the hands of teachers and managers, highlighting good performance to be shared and underperformance to be addressed. Teachers and managers can act on areas of concern in a much more timely fashion and before it is too late to affect change.

The scorecard can also shine a spotlight on those aspects of a teacher’s performance that are of particular importance, such as target-setting, assessment and feedback.

Leaders can access - just one-click from log-in - a “live” scorecard for their teams. We can ensure that every manager risk-assesses their scorecard at least monthly and has professional conversations with teachers where their scorecard poses questions. These are crucial - numbers can only tell us so much.

It’s also important that a scorecard permeates the whole organisation. Everyone should have one, including the principal and chief executive, to ensure consistency. No student should be allowed to fall through the net.

A balanced scorecard is only part of the solution, however. The best systems of quality assurance and performance management lead to formative feedback which helps teachers to improve their practice.

In other words, the best performance management processes do not merely draw lines in the sand but provide a roadmap to excellence, and are intrinsically linked to professional development. As such, in next week’s article, I’ll turn my attention to the importance of performance feedback.

Matt Bromley is an education journalist and author. He tweets @mj_bromley