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The participants repeat the experiment in each condition
Example: condition 1- listening to something when learning a list of pictures.
Condition 2- learning a list of pictures in silence.
Strengths: it reduces the number of individual differences because the
same people are used in each condition. This removes a possible confounding
variable improving the internal validity of the conclusions.
Weaknesses: it can reduce a different extraneous variable that of practice
or fatigue effects. To overcome this counter balancing can be used. Counter
balancing is when half of the participants do condition A first and then
condition B. The other half of the participants do condition B first and then
condition A. This balances out the practice and fatigue effects.
There are different participants in each condition.
Strengths: it removes the possible extraneous variables of practice and
fatigue and reduces the likelihood of demand characteristics as the
participants only take part in the experiment once and so are less likely to
determine the hypothesis.
Weaknesses: it can introduce a different extraneous variable that of
individual differences, e.g. the boys may do better as they may have a
The samples are matched into pairs depending on characteristics, similar
age, similar IQ or personality for example.
Strengths: it reduces the possibility of individual differences becoming
confounding variables and avoids practice or fatigue effects and reduces
the chances of demand characteristics.
Weaknesses: it can be very difficult to match the pairs appropriately, and
it is almost impossible to completely match participants on all types of
variables which affect their performance in the experiment.