The 2,300 teachers Labour says are needed to reduce classes to under 30 for five, six and seven-year-olds is a figure based on an education authority with a significant number of large primary classes.
The National Foundation for Educational Research, which Labour commissioned in June to carry out the research, chose an unnamed and low-spending local authority to study.
The NFER and the authority estimated that 100 teachers would be needed to reduce class sizes to 30 across its 400 primary schools. This figure was then extrapolated to all education authorities to give an estimate of 5,000 extra teachers nationally.
However, by concentrating on classes for five, six and seven-year-olds, the figure falls to 2,300.
Separate figures for the number of infants in classes of more than 30 are not available, according to the NFER.
The foundation says that in recent years an increasing number of schools have tried to reduce the size of infant classes. Hence in primary schools it is likely that there is a disproportionately lower number of infant classes of 30-plus. The NFER, therefore, weighted its estimates. It assumed that about 14 per cent of the total of all primary teachers needed would be enough to reduce class sizes in either Year 1 or 2, and it assumed that 18 per cent would be needed to reduce classes in Year 3. This means that, for each of the first two years, the estimated number needed is about 700. For Year 3, the number would be 900.
On staff costs, the NFER has estimated paying each teacher about Pounds 26,000, which includes 15 per cent for costs above salary. This leads to a staffing cost estimate of Pounds 59.8 million for Years 1 to 3.
However, the NFER admits there are two main shortcomings to the estimates: other costs such as accommodation for the extra classes have been ignored, and no account has been taken of how the schools would get the extra funding to reduce class sizes.