# Glossary

**Co-operative learning**

An approach to learning where pupils work together in small groups, usually of two or four children, to master a new concept or solve a problem. Pupils take responsibility for each otherâ€™s learning.

** ****Effect size **

The effect size shows how much difference a programme/intervention makes.

It is the difference between the mean (or average) of the *experimental group*, and the mean (or average) of the *control group*, divided by the *standard deviation* of the control group.

The important point is that the larger the effect size, the greater the difference the programme/intervention has made. An effect size of less than 0.2 is considered weak, an effect size of 0.2 to 0.4 is considered moderate, and an effect size of more than 0.5 is considered strong.

**Experimental groups/control groups **

Good scientific studies of education programmes will include an experimental group (sometimes called a treatment group) and a control group.

The experimental group are pupils who experience the programme/intervention being tested. The control group are pupils who continue to be taught as usual, but whose progress is monitored so that it can be compared to the progress of the experimental group.

** ****Matched study **

A type of study where *experimental and control groups* of pupils are matched on relevant criteria, for example on their average reading test scores. By ensuring the pupils are similar at the beginning of the experiment, any effect that the programme/intervention makes is clear.

**Mean effect size **

The average of a set of results, eg the average of pupilsâ€™ test scores. Also see *effect size*.

** ****Median effect size **

The middle number in a set of results. Also see *effect size*.

**Quasi-experimental research design **

A type of study where *random assignment* of pupils, classes or schools is not possible. The pupils in the *experimental and control groups* have therefore not been allocated randomly, and this must be taken into account when drawing conclusions from the results.

**Random study/random assignment **

* *A type of study where pupils are randomly allocated either to be part of the *experimental group *or the* control group*. Randomly assigning in this way avoids any bias.

**Standard deviation **

A measure of the dispersion of results around the mean (or average). Also see *effect size*.

**Weighted mean effect size **

The average *effect size* is interpreted in terms of the influence given to particular variables, in order to even out discrepancies. For example, a study may comprise 50 per cent boys and 50 per cent girls, but the schools involved have 60 percent boys and 40 per cent girls in reality. The average effect size would be weighted to compensate for this discrepancy.

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