A pilot study into the impact of class sizes on attainment at secondary schools in Scotland has raised important questions over whether there is a link between the two.

Research by Edinburgh University into the exam results of S4 pupils from North Lanarkshire in 2006 found that in the case of maths, attainment of pupils was significantly lower in smaller classes.

In chemistry and modern studies, the report found that class size "does not appear to have an effect on attainment".

Because of the limited nature of the study and the fact it provides only a snapshot of one year group, no definitive conclusions can be drawn. Nevertheless, the findings will be of great interest given the political focus on the importance of smaller class sizes and whether money to achieve them would be better spent on other initiatives.

Reducing class sizes has been forced on to the political agenda by the Educational Institute of Scotland (EIS), Scotland's largest teaching union, whose members have threatened to strike unless all classes in all subjects are reduced to 20.

The previous Scottish Executive, led by Labour and the Liberal Democrats, began the reduction by bringing class sizes down to an average of 20 in English and maths in the first two years of secondary.

The current Scottish Government has also set its own class size targets with plans to reduce them to 18 in the first three years of primary, with councils allowed to progress at their own pace.

However, the latest research, alongside other international studies, highlights uncertainty over whether there is enough evidence to suggest such policies get results. Rhona Brankin, education spokeswoman for Scottish Labour, said using teachers in innovative ways was more important.

"The danger is that if you use additional staff just to reduce class sizes you miss opportunities to use extra teachers to target specific problems such as poor literacy."