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Reduces participant variables because the researcher has tried to pair up the participants so that each condition has people with similar abilities and characteristics. 

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Experimental design refers to how participants are allocated to the different conditions (or IV levels) in an experiment.

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Pro: Reduces participant variables because the researcher has tried to pair up the participants so that each condition has people with similar abilities and characteristics.
Pro: Avoids order effects, and so counterbalancing is not necessary.

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.

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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.

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The researcher must decide how he/she will allocate their sample to these IVs.  For example, if there are 10 participants, will all 10 participants take part in both conditions (e.g. repeated measures) or will the participants be split in half and take part in only one condition each?

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the common way to design an experiment in Psychology is to divide the participants into two groups: the experimental group and the control group, and then introduce a change to the experimental group and not the control group

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3. Matched pairs:

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.

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Pro: Avoids order effects (such as practice or fatigue) you only participate in one condition only.  If a person is involved in more than one then they may become bored, tired and fed up by the time they come to the second condition, or becoming wise to the requirements of the experiment!

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).

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There are three types of experimental designs:

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Pro: Fewer people are needed as they take part in all conditions (saves time and money).
Pro: As the same participants are used in each condition, participant variables/individual differences are reduced.

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.

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1. Independent measures:

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.

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2. Repeated measures:
The same participants take part in each condition of the independent variable.