Step 7: Develop hypotheses

hypothesis is a statement, the "best guess" using the available information, regarding the explanation of the occurrence of some event. The purpose of a hypothesis is to provide a logical basis for planning and conducting the various investigations necessary to investigate an outbreak. Therefore, the hypothesis must be stated in such a way that it can be tested, and the results of the test provide a clear answer to the question of whether the hypothesis is correct.

The hypothesis should address the source of the agent, the mode (and vehicle or vector) of transmission, and the exposures that caused the disease.

Following the descriptive analysis of the outbreak data, the outbreak characteristics may be well understood. However, the conclusions possible from descriptive data are limited. Essentially, descriptive analysis and control measures enable a person to generate a hypothesis.


Descriptive studies generate hypotheses, while analytic studies test them.

The following questions can be asked to assist with the hypothesis generation:

  1. What is the agent's usual reservoir?

  2. How is it usually transmitted?

  3. What vehicles are commonly implicated?

  4. What are the known risk factors?

  5. What was the cause of the outbreak?

  6. Was transmission person to person or common source?

  7. How effective are the control measures?

The purpose of the analytic study is to gather further information to support or reject the hypothesis. There are occasions when one probably does not need to conduct an analytic study. If the clinical, laboratory environmental and/or epidemiologic evidence obviously supports the hypotheses, then formal hypothesis testing is probably unnecessary. For example, if a group of friends all eat mahi-mahi at a party and everyone gets scombroid poisoning and the toxin is lab confirmed in a leftover mahi-mahi, a formal study is probably not necessary.

Developing a Hypothesis

To develop a hypothesis:

  1. Identify the objective trying to be reached (for example, identify the source of exposure to implement successful control measures to prevent more cases).

  2. Identify the available information relevant to that objective. This information includes the signs, symptoms, laboratory findings of the reported cases, common causes, sources, reservoirs and modes of transmission of the condition under investigation, and the specific criteria established for a case. Discuss the cases with physicians, nurses, and local public health staff. Visit the home of the cases, on site and in the neighbourhood. Look for dominant patterns in the descriptive epidemiologic data.

  3. Develop a statement, based on the available information, and state it as a hypothesis in specific terms: these persons suspected as having disease "x" got their exposure from "y". If more than one logical conclusion can be reached, the investigator might concurrently establish an additional hypothesis. In a controlled scientific environment, it is generally preferable to establish and test one hypothesis at a timeHowever, the time constraints and difficulty in gathering information mean that we often have to state more than one hypothesis. In practical terms, however, this usually means having a list of suspect exposures (foods or locations).