Our experiment involves taking sheets of glass and coating them with sunscreen lotion of different SPF ratings, then placing the coated sheets of glass under bright sunlight and taking a UV reading behind the coated glass.
In our example, you would set up several identical experiments (called "controls") with only the SPF rating changed. The other variables for the experiment, such as the brand of the sunscreen lotion, the quantity of sunscreen lotion used for each experiment, the type of materials and the type of UV meter used for each experiment, and the temperature, lighting conditions and humidity of the place where the experiment is conducted, must be kept the same for every experiment. You have to perform the experiment within a small window of time, under the hot sun and when there are no clouds. You should also make sure to purchase all of the lotion from the same pharmacy, and to purchase the lotion at the same time (just in case different batches of the same brand of lotion are manufactured with different specifications). The same UV meter should be used for each control experiment, as different meters (even though they are of the same brand/model) may produce different results due to many possible reasons (for example, the batteries may have run out in one UV meter, but not the other).
All of these variables are called control variables. Notice that when you're designing the procedure for your project, you must include steps for measuring the results of all of the experiments.
All scientific theory must be substantiated by reproducible test results. These tests must be reproducible by other people. That is why it is so important for you to keep good, systematic records of your experiment!
As you perform your experiment, you need to watch out for the following types of errors or mistakes:
As you design your experiment procedure, you need to identify the independent variable, and how to measure how the changes in the independent variable affects the dependent variable. Bear in mind that there can only be one independent variable. All other variables must remain constant. This is the ensure that any changes in the dependent variable can be properly attributed to changes in the independent variable. In our example, we need to ascertain how changes in SPF ratings affect the level of protection. If you don't keep all other factors constant during the experiment, other than the SPF level (for example, if one control experiment was done under bright sunlight whilst another is done late in the evening during sunset), you won't be in the position to say that the results which you observed were caused by the changes in SPF levels. Your procedure should also specify how many trials are to be performed. At a minimum, you should repeat your experiment three times, although there's nothing to prevent you from repeating more. For experiments that involve surveys, there's no sense in asking the same person the same question more than once - however, for this type of experiment, you should ensure that you survey a large number of participants for more reliable results.