All variables, which are not the
independent variable, but could affect
the results (e.g. dependent variable) of
The variable which you measure
The variable which you change
Design used in experiments
Different participants are used in each condition of the
independent variable. This means that each condition
of the experiment includes a different group of
participants. This should be done by random allocation,
which ensures that each participant has an equal
chance of being assigned to one group or the other.
Strength : Avoids order effects (such as
practice or fatigue) as people participate
in one condition only. If a person is
involved in several conditions they may
become bored, tired and fed up by the
time they come to the second condition,
or becoming wise to the requirements of
Limitation : More people are needed than
with the repeated measures design (i.e. more
The same participants take
part in each condition of the
independent variable. This
means that each condition of
the experiment includes the
same group of participants.
Strength : Fewer people are needed
as they take part in all conditions (i.e.
Limitation : There may be order effects. Order
effects refer to the order of the conditions having
an effect on the participants’ behavior.
Performance in the second condition may be
better because the participants know what to do
(i.e. practice effect). Or their performance might be
worse in the second condition because they are
tired (i.e. fatigue effect).
One pair must be randomly assigned to the
experimental group and the other to the
Strengths : Avoids order effects, and so counterbalancing is not necessary.
Limitation : Impossible to match people exactly, unless identical twins!
The researcher decides
where the experiment will
take place, at what time,
with which participants, in
what circumstances and
using a standardised
procedure. Participants are
randomly allocated to each
Strength : It is easier to replicate
(i.e. copy) a laboratory experiment.
This is because a standardised
procedure is used.
Limitation : The artificiality of the setting
may produce unnatural behavior that does
not reflect real life, i.e. low ecological
validity. This means it would not be possible
to generalize the findings to a real life
Field experiments are done in the everyday (i.e.
real life) environment of the participants. The
experimenter still manipulates the independent
variable, but in a real-life setting (so cannot
really control extraneous variables).
Strength : Behavior in a field experiment
is more likely to reflect life real because of
its natural setting, i.e. higher ecological
validity than a lab experiment.
Limitation : There is less control
over extraneous variables that
might bias the results. This makes
it difficult for another researcher
to replicate the study in exactly
the same way.