Step 2 - Choosing the variables
|I am going to find out what happens to ...|
|... when I change ...|
|I am going to keep these the same (constant) to make it fair ... |
Once again the table is made available to the children. This may be put up on the wall as a poster or written on the board or copies given to group leaders or to all the pupils.
The group are asked to select one variable that they will change (independent variable) and one that they will measure (dependent variable). All the other variables must be kept the same if there is to be a fair test.
The concept of a fair test is crucially important in planning an investigation. The pupils should be taught to control the variables other than the dependent and independent variables in a conscious way. Often the more ‘obvious’ a variable is, the more likely it is to be controlled, but the pupils should be trained to consider their set-up and decide on the variables to be controlled.
A fair test is one in which only the independent variable is seen to cause a change in the dependent variable. If, for example, two things change, say temperature and humidity, you cannot be sure which of these causes the change in the dependent variable; it may be temperature or it may be humidity or it may be a combination of both.
- It is only by carrying out a fair test that you can be sure that it is what you have changed (independent variable) that is affecting what you measured (dependent variable).
- It is easier to recognise that a test is fair than it is to plan and carry out a fair test.
- You will need to encourage the pupils to make sure that all relevant aspects have been controlled (kept the same).
- Most pupils need only say that they intend to keep certain things the same, but the most able pupils should be encouraged to discuss what value each control variable should have.
Note: the words independent variable and dependent variable do not need to be taught at this stage!