Types of Variable

All experiments examine some kind of variable(s). A variable is not only something that we measure, but also something that we can manipulate and controlled for. To understand the characteristics of variables and how we use them in research, this guide is divided into three main sections. First, we illustrate the role of dependent and independent variables. Second, we discuss the difference between experimental and non-experimental research. Finally, we explain how variables can be characterised as dichotomous (or nominal), discrete (or qualitative/categorical/ordinal), or continuous (or quantitative/interval/ratio).

Dependent and Independent Variables

Imagine that a tutor asks 100 students to complete a maths test. The tutor wants to know why some students perform better than others. Whilst the tutor does not know the answer to this, she thinks that it might be because of two reasons: (1) some students spend more time revising for their test; and (2) some students are naturally more intelligent than others (see the statistical guide, Hypothesis Testing, for more information on how to set up such an experiment). As such, the tutor decides to investigate the effect of revision time and intelligence on the test performance of the 100 students. The dependent and independent variables for the study are:

Dependent variable:	Test mark (measured from 0 to 100)
Independent variables:	Revision time (measured in hours) Intelligence (measured using IQ score)

The dependent variable is simply that, a variable that is dependent on an independent variable(s). For example, in our case the test mark that a student achieves is dependent on revision time and intelligence. Whilst revision time and intelligence (the independent variables) may (or may not) cause a change in the test mark (the dependent variable), the reverse is implausible; in other words, whilst the number of hours a student spends revising and the higher a student’s IQ score may (or may not) change the test mark that a student achieves, a change in a student’s test mark has no bearing on whether a student revises more or is more intelligent (this simply doesn’t make sense).

Therefore, the aim of the tutor’s investigation is to examine whether these independent variables – revision time and IQ – result in a change in the dependent variable, the students’ test scores. However, it is also worth noting that whilst this is the main aim of the experiment, the tutor may also be interested to know if the independent variables – revision time and IQ – are also connected in some way.

In the section on experimental and non-experimental research that follows, we find out a little more about the nature of independent and dependent variables.

Experimental and Non-Experimental Research

In some cases, an independent variable(s) is manipulated in order to examine the effect that this change has on a dependent variable(s). In other cases, an independent variable(s) cannot be manipulated. This highlights the difference between experimental and non-experimental research.

• Experimental research: In experimental research, the aim is to manipulate an independent variable(s) and then examine the effect that this change has on a dependent variable(s). Since it is possible to manipulate the independent variable(s), experimental research has the advantage of enabling a researcher to identify a cause and effect between variables. For example, take our example of 100 students completing a maths exam where the dependent variable was the exam mark (measured from 0 to 100) and the independent variables were revision time (measured in hours) and intelligence (measured using IQ score). Here, it would be possible to use an experimental design and manipulate the revision time of the students. The tutor could divide the students into two groups, each made up of 50 students. In “group one”, the tutor could ask the students not to do any revision. Alternately, “group two” could be asked to do 20 hours of revision in the two weeks prior to the test. The tutor could then compare the marks that the students achieved.