Practical Skills


Shini Somara and Simon Clark present the steps that should be taken when planning a science investigation

When scientists start to investigate something they usually have a hypothesis that they are testing. This means they have an idea about what will happen when they explore something or take some readings, but they need the evidence to either confirm their thinking or suggest they need to think again.

From this they can make a prediction. It is easy to get mixed up between hypotheses and predictions. For example, a hypothesis might be made about the way that springs behave when they are loaded. From this a prediction can be made about what will happen to a spring when force is added.


Often an experiment involves things that can change, known as variables. Variables need to be identified, so they can then either be changed or controlled. There are three kinds of variable:

  • dependent
  • independent
  • control

Scientists often want to find out if changing one variable makes a difference to other variables. In many (though not all) investigations the variables are kept constant - the control variables, apart from one which is varied - the independent variable. The effects of the independent variable is then determined by monitoring the dependent variable.

An example would be investigating whether increasing the length of a pendulum might alter its time to make one complete swing (the period). As it is the length of the pendulum which is changing, that would be the independent variable. The changing length alters the time for one complete swing, therefore the period is the dependent variable. When carrying out the experiment, care has to be taken that other variables that affect the period, such as mass of the pendulum bob, are kept constant. These are control variables.

Values and readings

The values are the measurements used for the independent variable. If, for example, one of the variables in an experiment was length, it would be important to decide the maximum and minimum values, and also the intervals between values. If electric current at different voltages was being investigated, a decision would have to be made on what range of currents to use. This decision would take into account elements such as available equipment, the scales on the measuring instruments and safety.

When measurements are being taken, it is usually appropriate to repeat them. Sometimes, there are lots of possible readings that could be taken. For example if the length of a pendulum was being varied, it wouldn’t be necessary to measure every centimetre; however, it wouldn’t be a good idea to measure every 10 centimetres. To reduce human error when, for example, starting and stopping a stopwatch, repeat measurements should be taken. A mean is then calculated.