When investigating physical phenomena, there are two strands to thinking about variables:
- the difference between independent, dependent and control variables;
- the intrinsic nature of the variables, i.e. continuous, discrete, or categorical variables.
Independent, dependent and control variables
When planning an investigation, students need to identify the variables that they consider significant and then select the ones they want to look at for ‘cause and effect’. If possible, any other variables that will affect the outcome of the experiment need to be kept constant during the investigation.
Independent variable: The variable that is deliberately changed by the experimenter.
Dependent variable: An outcome variable that is measured each time the independent variable is changed.
Control variable: Any other variable that may affect the outcome. If at all possible, these need to be kept constant during the experiment. If they cannot be kept constant then their values need to be monitored or recorded so that their influence can be assessed.
The intrinsic nature of the variables
The nature of each variable affects how it can be observed or measured, and how the data collected is best displayed.
Continuous variable: A numerical variable that can have any number. If dependent and independent variables are both continuous, then the data can be represented by a line graph.
Discrete variable: A variable that involves simple numbers or a physical quantity with measured values that are quantized. For example: the number of springs connected together, the number of 100 g masses, or energy levels in atoms.
Categorical variable: Something that can be described by a label and not a number, e.g. colour, or material of construction. These can be displayed as a pie chart or bar chart.
Ordered variables: Some things can be put into size order, without being quantified. e.g. marble chips might be described as large, medium or small.