This involves different participants being used in each condition of the independent variable.  This means that each condition of the experiment includes a different group of participants. 

Con: More people are needed than with the repeated measures design (more time consuming and expensive).

Con: Differences between participants in the groups may affect results, for example; variations in age, sex or social background.  These differences are known as participant variables (i.e. a type of extraneous variable).

Con: There may be order effects. Performance in the second condition may be better because the participants know what to do (practice effect).  Or their performance might be worse in the second condition because they are tired (fatigue effect). This limitation can be controlled using counterbalancing.

The sample would split into two groups experimental (A) and control (B).  For example, group 1 does ‘A’ then ‘B’, group 2 does ‘B’ then ‘A’ this is to eliminate order effects. 

Each condition uses different but similar participants. An effort is made to match the participants in each condition in terms of any important characteristic which might affect performance, e.g. sex, age, intelligence etc.

One member of each matched pair will be randomly assigned to the experimental group and the other to the control group.

Con: Very time-consuming trying to find closely matched pairs.

Con: Impossible to match people exactly, unless identical twins!

Con: If one participant drops out you lose 2 PPs’ data.