Scientific Method

• There are four main parts to the scientific method. First, the scientist observes a phenomenon or set of phenomena. Then she comes up with a hypothesis to describe the phenomenon. A hypothesis is a proposed suggestion of why the phenomenon occurs and the scientist uses it as a starting point for further investigation. The scientist makes predictions using the hypothesis and then constructs an experiment to test these predictions. If the experiment confirms the hypothesis, and repeated experiments show the same thing, it eventually becomes a theory. A theory is a hypothesis that has yet to be proved wrong by experiment.
  
  

Experimental Variables

• The different quantities in a scientific experiment are known as variables. The variable that you change is known as the independent variable, and the variable that you measure is called the dependent variable. For example, if you were measuring the effect of exercise on heart rate, the independent variable would be the amount of exercise and the dependent variable would be heart rate. There will be other variables that you could change, but it is important to just change one at a time.
  
  

Keeping Variables Constant

• A constant is a variable that does not change. While conducting an experiment, you will need to keep several variables constant for the experiment to produce valid results. In the example where you change the amount of exercise and measure heart rate, for example, you would need to keep the fitness level of the person constant, as well as the type of exercise they do and and the conditions in which they are exercising.
  
  

Reasons for Keeping Variables Constant

• You need to keep all variables except the independent and dependent variables constant in order to produce results that you can use to test the initial hypothesis. This is because if other variables are not kept constant, there is no way to check which of the variables was affecting the outcome of the experiment. In the example, to conclude that exercise does or does not affect heart rate you must be sure that nothing except the amount of exercise has changed during the experiment.