The concept of using the appropriate control groups in an experiment is extremely important, and is THE most important fundamental concept to be learned by a graduate student. This is oftentimes included within the definition of the experiment itself. Every experiment should be repeated several times and, if possible, also in various ways. Random chance and unpredicted outside influences may affect the data, especially if only a single experiment is performed. Repetition helps eliminate errors and allows consistency.
Statistics is a highly useful tool in helping scientists keep a handle on variability.
Conclusions and modifications of the hypothesis . Oftentimes courses include the conclusions and modifications as a component of the experiment itself and, thus, they are usually not mentioned. Once the experiment is over, one analyzes the data and determines if the information supports the hypothesis. Often, the hypothesis will need to be modified or changed after the experiment. A theory is then a framework within which observations are explained and predictions are made.