The scientific method
Independent, dependent and control variables
An experiment will typically involve making changes to a system and measuring how the system is affected by those changes. Say, for example you are trying to investigate the deflection of a beam. There are lots of factors that might influence the amount of deflection, such as the force applied to the beam, the location the force is applied, the material from which the beam is made, the thickness of the beam...etc. As the outcome of your experiments should establish a model that can predict the behaviour of the system to each of these different factors, it is important you know how they individually contribute to changes observed. The scientific method is a design method that isolates the contribution of individual factors within an experiment.
Put simply, to determine how an experimental system will behave in response to only one factor, the scientific method involves altering ONLY that factor during the course of the experiment. All other factors that might alter the behaviour of the system should remain unchanged. In order to have a language to describe the scientific methods, three terms are defined:
Independent variable
This is the factor in the experiment that you set to see how the experimental system behaves when it is changed.
Dependent variable
This is the factor in the experiment that changes as a result of setting the independent variable. It is the factor which you measure or observe.
Control variables
These are all the other factors that could influence the behaviour of your experimental system. They are left unchanged during the experiment
Top Tip! Before conducting all experiments, it is useful to identify which variables are the dependent, independent and control variables and explicitly write this down in your plan and procedure.
The graph to the left show the typical presentation of a results from an experiment. The independent variable that you set would be plotted on the horizontal axis and the dependent variable that you measure would be on the vertical axis. The experimental measurements are shown as data points and the inferred relationship that describes the behaviour of the system shown with a trend line.
An example of applying the scientific method
As an example, consider employing the scientific method for the beam experiment above, which determines the amount of deflection created when a force is applied. If we set the force and measure the deflection, the independent variable would be the amount of force applied and the dependent variable would be the measurement of deflection. The control variables would be the position on the beam where the force is applied, the size, shape and material of the beam...etc which will all remain constant during the experiment.
The aim can help to decide which factors and in an experiment are the dependent and independent variables. If you are looking for a relationship between two parameters, the choice for which is which is can sometimes be fairly arbitrary. The decision might be based on practical considerations such as how the equipment you are using works.
Summary: An experiment will typically involve establishing how a system behaves in response to changing a certain parameter. To ensure that other factors do not influence the results, the scientific method is employed. This method involves determining which parameter you are going to set (the independent variable), which you are going to measure (the dependent variable) and ensuring all other parameters (the control variables) remain unchanged during the experiment. It is wise to establish which parameter is which and write these down during the process of planning any experiment.