The Scientific Method in Psychology

The picture below illustrates “the circle of science”. The scientific method uses deductive reasoning as a way of finding out about the world. When we use deductive reasoning we examine a variety of possibilities and work out which possibilities are wrong in order to get closer and closer to the “truth”. This is why psychologists say they have found evidence to support a specific possibility, rather than saying they have found proof. The whole process starts with psychologists making observations of interesting phenomena in the real world and asking questions about why this particular event, for example, has arisen or why people think, feel and behave as they do. Experimental psychologists then create scientific/testable theories or explanations and from these they generate hypotheses. These hypotheses (or “if-then predictions” about what will happen under specific circumstances) are then tested using experiments. If the experiment returns data which does not support the theory then theory will need to be altered or modified and then retested. This is why the scientific method is illustrated as a circle.


Experiments, IVs and DVs

Experimental studies attempt to demonstrate a causal relationship between two variables, that is, to show that one variable can cause an effect on another. In an experiment the researcher isolates and manipulates one variable (the independent variable or IV) and looks at whether this affects another variable, (the dependent variable). A causal relationship can only be established if all other variables that might affect the measured variable (dependent variable) are controlled (held constant). Any changes in the dependent variable (DV) can then be said to have been caused by the independent variable, (IV).

Operationalisation of variables

This long word simply means precisely defining your variables and in the case of the DV, making it measurable, in a way that can be as closely replicated as possible. When you define your IV, you must be clear about exactly what it is that is being changed between the two groups or conditions. When operationalising your DV , you need to ensure that you have commented on exactly how this variable should be measured and what the units would be, e.g. seconds, words free recalled in one minute, balls that hit a circular target (30 cm in diameter) from a distance of 5 metres. If the DV is a score representing an attitude on a ranked scale for example, you need to be sure to say that 1 is low and 7 is high, for example.

Practice Questions

  • Highlight the IV in one colour and the DV in another for each example. Remember there will be two parts to each IV (the two levels of the IV: the different conditions or groups)
  • For each example think about how you might operationalise the dependent variable, i.e. how will you measure it? What level of measurement will you be using (NOIR)?
  • What would you need to control in order to sure about cause and effect, thus enhancing the internal validity of the final conclusion.
  1. Rats that have lived in more stimulating environments will run mazes more quickly than rats that have lived in impoverished environments.
  2. People who listen to dance music in the gym will feel more motivated to exercise than people who listen to ballads in the gym.
  3. People who score high on the f-scale will be more willing to follow order to harm another fellow human than people who score low on the f-scale.
  4. Actors who learn their lines in the theatre where they will perform a play need less prompting in the final show than people who learn their lines at home.
  5. Liverpool fans are more likely to help to a man who trips in front of them when the man is wearing a Liverpool shirt compared with a plain T shirt.

Mr Faraz wants to compare the levels of attendance between his psychology group and those of Mr Simon, who teaches a different psychology group. The independent variable in this particular investigation is

A level of attendance in the two groups.

B whether the teacher is Mr Faraz or Mr Simon.

C the average level of attendance in each group.

D whether the teacher sets homework or not.

Different Types of Experiment

Laboratory Experiments

Laboratory experiments take place in controlled environments, where the researchers can ensure that situational variables, which might affect the DV, are held constant between trials, meaning that every participant has the exact same experience as every other (standardised procedure) and therefore any difference in the measured variable (the DV) between the two conditions or two groups must be due to the manipulation of the IV, enhancing the internal validity, (or certainty that the DV was affected by the IV).

Because lab experiments have a standardised procedure they can also be replicated to check the consistency of the findings. This mean that the results can also be said to be reliable.

Field Experiments

Field experiments also aim to demonstrate a causal relationship between an IV and a DV however they taker place in natural settings where you might expect to find people going about their everyday lives. Often the people in field experiments do not even realise that they are in an experiment. This means that the ecological validity is improved, that is, the extent to which findings can be said to be useful in telling us about real world behaviour. This is not always the case in lab experiments where the tasks that people are asked to do (and the settings in which they are behaving) can be described as contrived or artificial, in that they have been designed specifically for the purpose of the study and do not always bear any relation to the every day lives of the people in the study. In a field experiment, the researcher still manipulates an IV often setting up a specific situation but in a real world environment and then comparing the behaviour in this set up situation to behaviour in some other condition. Moreover, they may look at how different groups of people respond to the same set up situation in the real world.

As you can probably start to realise there a number of advantages and disadvantages to both lab and field experiments which you will have the opportunity to explore in the exercises below, suffice to say for now that field experiments can have problems with regards to internal validity and reliability, can you think why?

Natural Experiments

Natural experiments is the name given to an experiment when the IV is naturally occurring, that is it has not been manipulated by the experimenter. For example,  in a study by Becker they looked at the rate of eating disorders before and after the introduction of Western TV channels in Fiji. Similarly, Charlton they looked at rates of pro and antisocial playground behaviour in children before and after TV was introduced to the remote island of St Helena. The researchers did not choose to deprive a group of people of exposure to Western TV and then decide to provide access to the TV, this was a naturally occurring situation. Natural experiments can be an excellent choice when it is not practical or ethical to manipulate a certain variable, however there are problems with this sort of study as due to the researcher not having control of the independent variable and therefore not being able to employ methods such as  counterbalancing or random allocation to overcome problems in the design, the conclusions can be said to lack internal validity. Sometimes natural experiments take place in lab settings and sometimes in a more natural environment; the type of environment is not a feature of the natural experiment and may different from study to study.

Again, you should be able to think up some other advantages and disadvantages of this style of experiment.


Edexcel students do not need to know this term but IB students do. You will see this term being used in various different ways in different texts. Over the years, the meaning has evolved and nowadays people use it to refer to something rather different to its original meaning! The IB expect you to use it as described below.

Essentially, the prefix “quasi” means “similar to” so a quasi experiment is similar to a lab experiment but not quite. In a quasi experiment participants cannot be randomly allocated to groups, this is because the IV relates to some feature of the people taking part, e.g. culture, gender, personality, intelligence, preferred hand (left or right) attitudes towards a certain thing etc. Quasi experiments are therefore also like natural experiments in that the IV can be said to be naturally occurring and not under the control of the researcher, however, the term quasi is used rather than natural because the IV is not actually something that can physically vary for the individual, they are either male or female, right-handed or left-handed, American or Japanese etc.


Draw Venn diagrams with overlapping circles to compare the different types of experiment from your specification. Use this to draw out similarities and differences relating to reliability, internal validity, external validity, ethics and generalisability.

Practice Questions

Remember, for the following questions, comparisons MUST ALWAYS include similarities AND differences.

  1. Compare laboratory experiments with natural experiments.   (4)
  2. Compare field experiments with natural experiments. (4)
  3. Compare lab experiments with field experiments. (4)

Practice Questions:

Julia is studying psychology at university. As part of her course she has been asked to design and carry out an experiment that looks at the effects of alcohol on reaction times. Describe a procedure that Julia might use when experimenting on the effects of alcohol on reaction times. You must justify at least two of the decisions made. (6)

You might wish to consider the following:

  • Experimental design
  • Variables
  • Apparatus
  • Sampling
  • Ethics.