A theory accounts for many facts and attempts to explain a great variety of phenomena. A good theory shows the relationship among classes of facts and clarifies our understanding of natural phenomena. A theory generates many predictions, all of which have been supported. A good scientific hypothesis should generate predictions that can tested by objective measurements or observations using our senses. In many cases, the predictions are tested by doing experiments.
Controlled experiments are an important feature of the scientific process.
A controlled experiment allows conclusions to be made about the effect of experimental manipulation. Setting up appropriate controls is a key element of good experimental design. People who take Vitamin C will catch fewer colds. Other factors to consider in designing an experiment. Many other factors must be considered in designing an experiment.