Participants are matched as pairs on the basis of relevant variables. One of the pair is allocated to the experimental group and the other to the control group
A limitation of this experimental design is that participants can never be perfectly matched in every respect. The process of matching participants is also very time consuming and can be difficult to achieve
Researchers may therefore use the experimental procedure of counterbalancing to eliminate possible order effects such as fatigue, practice effects and boredom. Thus the experimental tasks are performed in different sequences by different participants or by the same participants at different times.
The placebo effect can be minimised by using a single blind procedure when allocating participants to groups.
In a single blind procedure, the participants do not know whether they are in the control group or experimental group condition. This prevents the participants’ knowledge or expectations of the experiment impacting on the results.
This refers to the way an experimenter (rather than the IV) might influence the dependent variable, and therefore the results of an experiment
One way is through experimenter bias , also referred to as experimenter expectancy effect. This is where the experimenter has certain expectations about the outcome of the experiment, which can subtly alter his/her behaviour and bias the results.
In order to minimise experimenter effects, researchers may use a double blind procedure. In a double blind experiment, neither the experimenter nor the participants know whether the participants are assigned to the experimental or control group until after testing or the conclusion of the experiment
A third person controls the placement of groups and never has contact with the participants.
This controls the Placebo effect and the Experimenter effect.