What is it called when the effect of one independent variable depends on the level of the other independent variable?

1. Experiments
2. Independent and Dependent Variables

By Dr. Saul McLeod, updated 2019

Variables are given a special name that only applies to experimental investigations. One is called the dependent variable and the other the independent variable.

The independent variable is the variable the experimenter manipulates or changes, and is assumed to have a direct effect on the dependent variable. For example, allocating participants to either drug or placebo conditions (independent variable) in order to measure any changes in the intensity of their anxiety (dependent variable).

In a well-designed experimental study, the independent variable is the only important difference between the experimental (e.g. treatment) and control (e.g. placebo) groups.

The dependent variable is the variable being tested and measured in an experiment, and is 'dependent' on the independent variable. An example of a dependent variable is depression symptoms, which depends on the independent variable (type of therapy).

In an experiment, the researcher is looking for the possible effect on the dependent variable that might be caused by changing the independent variable.

Examples of Independent and Dependent Variables in Experiments

For example, we might change the type of information (e.g. organized or random) given to participants to see what effect 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).

Activity

For the following hypotheses name the IV and the DV.

1. Lack of sleep significantly affects learning in 10-year-old boys.IV............................................................

DV...........................................................

2. Social class has a significant effect on IQ scores.

IV............................................................

DV.......................................................…

3. Stressful experiences significantly increase the likelihood of headaches.

IV............................................................

DV...........................................................

4. Time of day has a significant effect on alertness.

IV............................................................

DV...........................................................

Operationalizing Variables

To ensure cause and effect is established it is important that we identify exactly how the independent and dependent variables will be measured, this is known as operationalising the variables.

Operational variables (or operationalizing definitions) refer to how you will define and measure a specific variable as it is used in your study. This enables another psychologist to replicate your research and is essential in establishing reliability (achieving consistency in the results).

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 older 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.

Activity

For the following hypotheses name the IV and the DV and operationalise both variables.

1. Women are more attracted to men without earrings than men with earrings.

I.V._____________________________________________________________

D.V. ____________________________________________________________

Operational definitions:

I.V. ____________________________________________________________

D.V. ____________________________________________________________

2. People learn more when they study in a quiet versus noisy place.

I.V. _________________________________________________________

D.V. ___________________________________________________________

Operational definitions:

I.V. ____________________________________________________________

D.V. ____________________________________________________________

3. People who exercise regularly sleep better at night.

I.V._____________________________________________________________

D.V. ____________________________________________________________

Operational definitions:

I.V. ____________________________________________________________

D.V. ____________________________________________________________

How to reference this article:

McLeod, S. A. (2019, August 01). What are independent and dependent variables. Simply Psychology. www.simplypsychology.org/variables.html

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