Class sizes are one of the key drivers of budget decisions so you need to have a good grasp of them. Which is easier said than calculated, as Sam Ellis explains...

Mean numbers

"I’ve put you down for one of the small year 10 groups in next year’s timetable, is that OK?” asked John, the head of science. The small group turned out to be a collection of 16 recidivists who I nicknamed ‘the halogens’ after their tendency to react. Although notionally the ‘bottom set’ they were very mixed in ability, despite their homogeneous volatility. Insane Wayne actually gained a science double award grade C at the end, outperforming many of his peers in so-called higher groups.

The year group had 154 pupils in it. Science was taught in six groups. Six staff with 154 students gives an average class size of 25.6. But no one actually teaches the average class and in this example certainly not me. The top set contained 34; the groups then numbered 32, 25, 25 and 22, leaving me with my 16.

Accounting for different numbers of groups in the same year is a bit more involved. One option block had eight staff in it. Its average class size was 154÷8=19.25. Blocks containing some technology choices had ten staff, giving an average class size of 154÷10=15.4.

It raises two questions: How can I work out the overall average class size for the year taking account of all the different timetable groupings? Why would I want to know it anyway?

I’ll answer the second question first. The overall average class size is a curriculum statistic analogous to a medical measurement, like blood pressure. And like blood pressure, you need to understand what the numbers mean to get a view of how the patient is doing. It is also one of the seven variables that drive your budget.

The curriculum structure for the year group of 154 students was as follows in the chart below:

The total number of class periods is 184. But some of the groups may have more than one teacher allocated to them for the same period. The teachers might split the group or they may work as a team. It makes no difference to the overall number of groups in the table above but if you calculate the total in teacher periods it does.

Allocating a pair of teachers to each of the ICT groups, for example, would give a total number of teacher periods of 190, as opposed to the group period total of 184. The Analysis screen in the NOVA-T6 software gives the number of class periods. Care needs to be taken in assuming that this is the same as the total of teacher periods.

This leads to two versions of average class size. The version I am using is the one based on the total number of teacher periods. A calculation based on class periods is used in NOVA-T6 which, as I say, can lead to a different value.

How do I calculate the average class size for the year group based on the teacher periods? First, take the teacher periods allocated to the year group and divide that by the number of periods in the timetable week or cycle. In the example above, if there were no double staffed groups it would be 184÷25=7.36. This number tells me the average number of teachers working with the year on any one lesson in the week.

Now divide the total number of pupils in the year by that answer to get 154÷7.36=20.9. This is the average class size. It is also the operational ratio of pupils to teachers in that area of the timetable. As a type of blood pressure measurement, I would be looking for average class sizes in the 24 to 27 region in Key Stage 3 and 19 to 22 in KS4.

In the next issue, I will look at post-16, the link with the budget and whole-school periods like learning support and inclusion – where a member of staff is stapled to the naughty table with a pupil who has been temporarily exported from a group like the halogens.

Meanwhile, for more detail and associated spreadsheets go to www.ascl.org.uk/resources/library/contents

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