Manipulation and Control of Variables (AQA AS Psychology): Revision Note

Independent & dependent variables

• In an experiment, a variable is any factor that could change or be changed

• There are different types of variables within an experiment

  • The independent variable: the only variable that should be changed or manipulated throughout an experiment

  • The dependent variable: the variable that is measured to determine the outcome of an experiment (the results)

Independent variables

• The IV is required to observe the effect it has on the dependent variable (DV), which is the variable being measured

• Examples of independent variables:

  • Whether participants learn a list of 20 words in silence or learn the words in the presence of loud rock music

    • The IV = silence or loud rock music

  • Whether participants complete a jigsaw puzzle after 30 minutes of exercise or complete the puzzle after no exercise

    • The IV = 30 minutes of exercise or no exercise

  • Whether participants have been exposed to an aggressive model or have not been exposed to an aggressive model

    • The IV = aggressive model or no aggressive model

• A laboratory experiment must use an IV that has been implemented by the researcher e.g. loud music or silence

  • The IV cannot be naturally occurring e.g. gender, age, ethnicity

  • Any experiment which uses a naturally occurring IV is known as quasi/natural experiments

Dependent variables

• The dependent variable is the factor which is measured in the study to assess the effect of the IV

• Examples of dependent variables:

  • The number of items recalled from a list of 20

  • The time taken in seconds to complete a jigsaw puzzle

  • The number of imitative acts of aggression performed in a 20-minute observation schedule

• The DV depends on how the IV has been set up

  • If the IV is memorising items with loud music or in silence then it makes sense that the DV is ‘the number of….’ or ‘the time taken to…’

  • Thus the DV must be measured using quantitative data

• It is impossible to have a qualitative DV:

  • The measure will always take some form of numerical data, which can then be displayed in a graph and analysed statistically

  • Qualitative research collects data in the form of words

Operationalising variables

• Operationalising variables refers to how both the IV and the DV are put to work/implemented by the researcher (see also revision notes on hypotheses)

• Operationalising variables involves specifics on how each variable is to be manipulated (IV) and measured (DV)

Operationalising the IV

• To operationalise the IV the researcher needs to set up and define each condition so that it is clear that a difference between the conditions is investigated

• E.g. participants in condition 1 drink 200ml of a caffeinated drink prior to a memory test; participants in condition 2 drink 200ml of water before a memory test

  • The IV is broken down into two separate conditions, (200ml of caffeine or 200ml of water), each of which is clear from the wording provided

Operationalising the DV

• To operationalise the DV the researcher needs to design a procedure which enables relevant and appropriate data to be recorded/collected per participant with no ambiguity involved

• E.g. the number of correctly recalled items from a list of 15 words

  • The DV provides the number of total possible correct answers (15) and identifies how participant performance is to be measured (number of correctly recalled items) with no ambiguity involved

Extraneous variables

• Extraneous variables are any factors that intrude upon and adversely affect the DV (not the IV, which the researcher has specifically implemented) e.g.

  • time of day

    • some participants may be 'morning people' which means that they are likely to be more alert first thing compared to others

  • temperature

    • if the experimental space is too warm/cold it may affect participant performance on the task

  • mood

    • one participant may have just had a row with their partner or won some money or feel a bit ‘off’ that day: any of these events mean that their mood is likely to affect their performance on the task

• Extraneous variables are usually controlled so they have the same effect across all conditions, which should stop them from affecting the DV, e.g.

  • time of day

    • run the experiment at midday which should hopefully be a neutral time for everyone

  • temperature

    • ensure the experimental space has an ambient temperature 

  • mood

    • treat participants in a pleasant but neutral way

• It would be impossible to control every potential extraneous variable during the experimental phase but it is the researcher's responsibility to do all that they can to ensure that their research is objective and unbiased