Some of the most popular guidance reports my own organisation Leerpunt has published discuss differentiation in education. This reminded me of a study I read a while ago that hadn’t become a blog post until now. The question the researchers posed in their paper seems quite simple at first glance: what do you, as a teacher, actually need to know and be able to do to differentiate effectively?

However, the answer turns out to be much more extensive than one might think. After all, differentiation is not simply about some students receiving some extra explanation while others do more difficult exercises. If only it were that simple. Before you can make such kind of a decision, you must first:

• know where students stand in their learning process,
• where they need to go next, and
• why they get stuck somewhere.

Next, you need to know which adaptations might help them move forward. During the lesson, you must also be able to check whether that adjustment is working and make further corrections if necessary. Tired already?

All of that is precisely what Kyra Meutstege and colleagues tried to uncover. To do so, they used a cognitive task analysis . Instead of simply asking teachers which forms of differentiation they use, they observed eleven Dutch secondary school mathematics teachers who were considered above average in differentiation. Subsequently, they interviewed these teachers about what they had done during their lessons and, especially, why. The results were then discussed and supplemented by the views of subject-matter experts.

From all these analyses, The analysis identifies three types of knowledge teachers need in order to differentiate effectively::

1. The first one is obvious: you need to know your students. But by this, the researchers do not necessarily mean knowing what hobbies they have or what music they listen to. Above all, you need to know where they stand in their learning. What prior knowledge do they have? What have they already mastered? Where do they get stuck? Additionally, it helps to know how independent and motivated students are. Importantly, this knowledge is not something you gather just once, as it can evolve. During your lessons, you continuously gather information about your students.
2. Next, you need didactic and pedagogical knowledge. After all, knowing that a student does not understand something is only the beginning. You also need to know what you can do next to further help that student.
3. And finally, you need a considerable amount of subject knowledge. And we are not just talking about knowledge of the subject matter itself, but also about pedagogical content knowledge: knowing how to explain specific material, how the learning trajectory is structured, and which tasks are appropriate at different points. What we would even dare to call the translator-interpreter function of a good teacher. That knowledge of the learning trajectory is particularly important. If a student has trouble understanding something, you need to know which step you can go back to. If a student is already further down his or her learning process, you need to know what meaningful next step is possible.

This brings me to a first insight that I believe we sometimes forget when discussing differentiation. Effective differentiation requires strong subject knowledge on the part of the teacher. You can only meaningfully deviate from the path when you know that path and potential side paths exceptionally well yourself.

But knowledge is not enough. As a teacher, you also need to be able to put that knowledge to use!

The researchers describe four phases for this:

• preparing a series of lessons,
• preparing a separate lesson,
• the teaching itself, and
• evaluating the lesson.

These four phases form a continuous cycle.

All this means, therefore, that differentiation begins well before you enter the classroom. When preparing a series of lessons, for example, teachers try to gain insight into what their students already know and can do. To do this, they combine information from tests, previous observations, and information from colleagues. Among other things, they will check whether students have achieved previous learning objectives necessary for what is to come.

It all becomes more specific when you start preparing a lesson:

• What is the starting point for these students?
• What is the goal of this lesson?
• What explanations and assignments do we need to get them there?
• For which students might I need to provide something extra or different?

But even perfect preparation cannot predict what will actually happen in a classroom, as any experienced teacher will tell you.

Therefore, to me, monitoring seems to be the most important skill in the article’s overview. During the lesson, as a teacher you constantly monitor whether students understand what is happening and whether they are making progress. Based on these observations, you adjust your instruction where needed. Sometimes you will give a smaller group some extra explanation. At other moments you will need to explain something again to the whole class. And in the meantime, you keep gathering new information about the students that will be useful for future decisions.

This also makes clear why a rather static form of differentiation can become problematic. You can, of course, group students based on previous performance. But their need for support is not fixed forever. A student may need extra help with one learning objective while making faster progress on another. Differentiation therefore requires a continuous reassessment. This also fits rather nicely with what we know about high expectations!

After the lesson, however, you are certainly not finished as a teacher. The teachers evaluate whether students have achieved the goal and consider what might need to be changed in the next lesson. This happens for both the class group and individual students. For example, if it turns out that the extended instruction was still too difficult for some students, you must take that into account for the next lesson.

And no, we are not finished yet. The teachers in the study also looked back at the entire series of lessons and used what they had learned when teaching the same material the following year. Did I go too fast here? Did my explanation actually work? Where should I spend more time next time?

Put all of this together and differentiation comes down to a series of questions:

• Where is this student now?
• Where do they need to go?
• What do they need to get there?
• Is what I’m doing actually helping?
• And what does what I’m seeing tell me to do next?

The answers will differ from student to student and may change over time. You probably didn’t need this entire list to know that differentiation is difficult, but seeing it all together does make the problem rather obvious. It isn’t enough to notice that a student needs something different. You need to figure out what they need, address it while still teaching everybody else, see whether it worked, and then decide what to do next.

There is one more thing in the study that I think is worth mentioning. All of this doesn’t depend only on the individual teacher. Some circumstances simply make differentiation harder than others. Some examples are pretty obvious:

• Teaching a larger class makes it harder to keep track of individual students.
• More variation between students makes the task more complicated.
• Differentiation becomes harder when students are less able to work independently.

Then there are some very practical constraints:

• Less preparation time means less time to prepare alternative explanations or tasks.
• Longer lessons give you more room to differentiate than shorter ones.
• Seeing the same students more often makes it easier to understand what they need.
• Working with a small group is simply harder in a crowded room.

Schools can make some of this easier too. Good information about student progress helps teachers decide what to do. And if teaching materials already contain useful tasks or explanations at different levels, teachers don’t have to create everything themselves.

I’ve noticed more than once that teachers can get quite irritated when somebody tells them they should differentiate more. I think this study helps to explain why this is so often the case. Differentiation is an enormously complex professional skill. And whether teachers can do it well depends not only on what they know and can do, but also on the circumstances in which we ask them to do it.