In scientific experimentation, scientists seek to discover facts about the world around us and then group those facts into theories or laws which clarify the relationships between the facts. From the observations they make and data they collect, they try to explain an event or observation using the information that is currently available. This attempted explanation is called a hypothesis, and two hypotheses can be stated during the testing – the null hypothesis and the alternative hypothesis.

Usually, the null hypothesis and the alternative hypothesis represent two contradictory statements. They are said to be mutually exclusive; they can’t both be true. Also, when the null hypothesis and alternative hypothesis are stated, they should be exhaustive. The proposals should cover all the possible outcomes of the experimental investigation.

The goal of any scientific experiment is to disprove one of the hypotheses, usually the null hypothesis. Often, it is difficult, if not impossible to prove a hypothesis is correct, so instead scientists try to disprove all the other theories. At the end of an experiment, one of the hypotheses is said to be rejected or not rejected. By disproving, or rejecting the other hypotheses, they will eventually be left with one that has not been disproven.

In most instances, the scientist is trying to disprove the null hypothesis. The null hypothesis is often tentatively held to be true. It is usually the case where there is no change due to the variable being tested by the scientist. The alternative hypothesis states the opposite conclusion compared to the null hypothesis. The alternative hypothesis is a new or revised belief about a current situation based on the fact that the current belief is mistaken.

As an example, a drug company may be testing the results of a new cholesterol treatment. In this case, the null hypothesis would be that there is no change to the patient’s cholesterol level after taking the drug. Of course, it is unlikely that the drug company wants this to remain true and in fact the researchers are trying to collect data that will allow them to reject this hypothesis. The alternative hypothesis for this experiment could be that the drug does have some effect on the patient’s cholesterol level. This is a very general hypothesis, so it is likely that they will be working towards a much more specific alternative hypothesis, such as the drug treatment significantly reduces the cholesterol level of the patient.