Independent, Dependent and Extraneous Variables
by Saul McLeod published 2008
A variable is anything that can vary, i.e. changed or be changed, such as memory, attention, time taken to perform a task etc.
Variable are given a special names that only apply to experimental investigations. One is called the dependent variable and the other the independent variable.
In an experiment, the researcher is looking for the possible effect on the dependent variable that might be caused by changing the independent variable.
• Independent variable (IV): Variable the experimenter manipulates (i.e. changes) – assumed to have a direct effect on the dependent variable.
• Dependent variable (DV): Variable the experimenter measures, after making changes to the IV that are assumed to affect the DV.
For example, we might change the type of information (e.g. organized or random) given to participants to see what affect this might have on the amount of information remembered.
In this particular example the type of information is the independent variable (because it changes) and the amount of information remembered is the dependent variable (because this is being measured).
It is very important in psychological research to clearly define what you mean by both your IV and DV.
Operational variables (or operationalizing definitions) refers to how you will define and measure a specific variable as it is used in your study.
For example, if we are concerned with the effect of media violence on aggression then we need to be very clear what we mean by the different terms. In this case, we must state what we mean by the terms “media violence” and “aggression” as we will study them.
Therefore, you could state that “media violence” is operationally defined (in your experiment) as ‘exposure to a 15 minute film showing scenes of physical assault’; “aggression” is operationally defined as ‘levels of electrical shocks administered to a second ‘participant’ in another room’.
In another example, the hypothesis “Young participants will have significantly better memories than old participants” is not operationalized. How do we define "young", “old” or "memory"? "Participants aged between 16 - 30 will recall significantly more nouns from a list if twenty than participants aged between 55 - 70" is operationalized.
The key point here is that we have made it absolutely clear what we mean by the terms as
they were studied and measured in our experiment.
If we didn’t do this then it would be very difficult (if not impossible) to compare the findings of different studies into the same behavior.
Operationalization has the great advantage that it generally provides a clear and objective definition of even complex variables. It also makes it easier for other researchers to replicate a study and check for reliability.
When we conduct experiments there are other variables that can affect our results, if we do not control them. The researcher wants to make sure that it is the manipulation of the independent variable that has changed the changes in the dependent variable. Hence, all the other variables that could affect the DV to change must be controlled. These other variables are called extraneous or confounding variables.
Extraneous variables – These are all variables, which are not the independent variable, but could affect the results (e.g. dependent variable) of the experiment.
Extraneous variables should be controlled were possible. They might be important enough to provide alternative explanations for the effects.
There are four types of extraneous variables:
1. Situational variables – These are aspects of the environment that might affect the participant’s behavior e.g. noise, temperature, lighting conditions etc. Situational variables should be controlled so they are the same for all participants.
Standardized procedures are used to ensure that conditions are the same for all participants. This includes the use of standardized instructions
2. Participant / Person variables – This refers to the ways in which each participant varies from the other, and how this could effect the results e.g. mood, intelligence, anxiety, nerves, concentration etc.
For example, if a participant that has performed a memory test was tired, dyslexic or had poor eyesight, this could affect their performance and the results of the experiment. The experimental design chosen can have an affect on participant variables.
Situational variables also include order effects that can be controlled using counterbalancing, such as giving half the participants condition A first, while the other half get condition B first. This prevent improvement due to practice, or poorer performance due to boredom.
Participant variables can be controlled using random allocation to conditions of the independent variable.
3. Experimenter / Investigator Effects – The experimenter unconsciously conveys to participants how they should behave - this is called experimenter bias.
The experiment might do this by giving unintentional clues to the participants about what the experiment is about and how they expect them to behave. This affects the participants’ behavior.
The experimenter is often totally unaware of the influence which s/he is exerting and the cues may be very subtle indeed but they have an influence nevertheless.
Also, the personal attributes (e.g. age, gender, accent, manner etc.) of the experiment can affect the behavior of the participants.
4. Demand characteristics – these are all the clues in an experiment which convey to the participant the purpose of the research.
Participants will be affected by: (i) their surrounding; (ii) the researcher’s characteristics; (iii) the researcher’s behavior (e.g. non-verbal communication), and (iv) their interpretation of what is going on in the situation.
Experimenters should attempt to minimise these factors by keeping the environment as natural as possible, carefully following standardised procedures. Finally, perhaps different experimenters should be used to see if they obtain similar results.
Suppose I wanted to measure the effects of Alcohol (IV) on driving ability (DV) I would have to try to ensure that extraneous variables did not affect the results. These variables could include:
• Familiarity with the car: Some people may drive better because they have drove this make of car before.
• Familiarity with the test: Some people may do better than others because they know what to expect in the test.
• Used to drinking. The effects of alcohol on some people may be less than on others because they are used to drinking.
• Full stomach. The effect of alcohol on some subjects may be less than on others because they have just had a big meal.
If these extraneous variables are not controlled they may become confounding variables, because they could go on to affect the results of the experiment.
How to cite this article:
McLeod, S. A. (2008). Independent, Dependent and Extraneous Variables. Retrieved from http://www.simplypsychology.org/variables.html
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