Experimental Method


An experiment is a study of cause and effect.  It differs from non-experimental methods in that it involves the manipulation of one variable, while trying to keep all other variables constant.


Laboratory Experiments


In psychological experiments we try to keep all aspects of the situation constant except one - the one we are looking at.  For example, suppose we want to investigate which of two methods is more successful at teaching children to read.  The aspect that we vary is called the independent variable (IV) and we change this in a very precise way.  In this example the teaching method is the independent variable. We call the factor which we then measure, in our example it would be some measure of the children’s reading ability, the dependent variable (DV), because, if our ideas are correct, it depends on the independent variable.  In our example, the children’s reading ability depends on the teaching method used.


The variable which is being manipulated by the researcher is therefore called the independent variable and the dependent variable is the change in behaviour measured by the researcher.

All other variables which might affect the results and therefore give us a false set of results are called confounding variables (also referred to as random variables).  Examples of confounding variables in the example given might include the following


·        Differences in the instructions given by an experimenter or in the stimulus materials being used (which could be overcome by standardising instructions and materials foe all those taking part)


·        Differences between participants, e.g. in their age (which could be eliminated as a variable by using a single age group, or alternatively it could be made more constant by ensuring that the age structure of each of the groups taking part in the experiment is very similar).


By changing one variable (the IV) while measuring another (the DV)  while we control all others, as far as possible, then the experimental method allows us to draw conclusions with far more certainty than any non-experimental method.  If the IV is the only thing that is changed then it must be responsible for any change in the dependent variable.


The Field Experiment:


Sometimes it is possible to carry out experiments in a more natural setting, i.e. in ‘the field ’.  A famous example of this is the series of studies carried out by Piliavin et al (1969) in which they arranged for a person to collapse on an underground train and waited to see how long it was before the person was helped.  One of the independent variables they used was the appearance of the ‘victim’:  whether he was carrying a walking stick or whether he appeared to be drunk.

As with the laboratory experiment, the independent variable is still deliberately manipulated by the researcher.

However it is not possible to have such tight control over variables in the field, but it does have the advantage of being far less artificial than the laboratory.




Natural Experiments:


In some circumstances, psychologists can take advantage of a natural situation in order to carry out an investigation in circumstances which they cannot themselves manipulate.  For example, a primary school may decide to try out a completely new reading scheme and the effects of this could be compared with a similar school using a different reading scheme.  A local hospital may decide to have mixed wards rather than separate wards for men and women.  The effects on the patients of being in these wards could be compared with those in single-sex wards.


This is not a true experiment because the psychologist is unable to manipulate or control variables.  For this reason it is sometimes referred to as a quasi-experiment.  It is possible, though to compare two groups, the equivalent of an experimental and a control group.  It has the advantage that the participants are unaware that they are taking part in an investigation and it is certainly not as artificial as a laboratory setting.