The Experiment VariablesOur 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.
Watch out!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:
1. Errors in the measuring instruments that you are using.
Because this type of error will always produce a measurement that is higher or lower than the "true" value, it is called random error.
2. Systematic errors (also known as non-random errors), which are due to factors which bias the results of your experiment in one direction.