Step 4: Test Hypotheses

A hypothesis is tested to determine if the outbreak source has been correctly identified. Investigators use many methods to test their hypotheses. Two main methods are analytic epidemiologic studies and food testing.

Epidemiologic Studies

The most common type of study conducted during foodborne outbreaks are case-control studies. Investigators collect information from sick people (cases) and non-sick people (controls) to see if sick people were more likely to have eaten a certain food or to report a particular exposure. Case-control studies try to include controls who have had the same opportunities to be exposed to an unsafe food item as a case has. To achieve this goal, controls are carefully chosen using matching or selection techniques.

Statistical Testing

If eating a particular food is reported more often by sick people than by well people, it may be causing illness. Investigators can determine the strength of the association between food and illness in an epidemiologic study by using statistical tests. Investigators also use statistical tests to decide if more than one food might be involved in the outbreak.

Factors Considered When Interpreting Study Results

  • Frequencies of exposure to a specific food item
  • Strength of the statistical association
  • The food’s production, distribution, preparation, and service

Key words:

Hypothesis: A suggested explanation for the cause of a foodborne outbreak

Case: A sick person

Control: A non-sick person

Strength of association: How likely it is that illness has occurred due to the food rather than chance

Stealth food: Foods that people are unlikely to remember eating. Examples: garnishes, condiments, and ingredients that are part of a food item

Photo of a lab testing food.

Food Testing

Food testing can provide useful information and help to support a hypothesis. Finding bacteria with the same DNA fingerprint in an unopened package of food and in the stool samples of sick people can be convincing evidence of a source. However, relying only on food testing can also lead to results that are confusing or unhelpful.

When is food testing not usually helpful?

  • Food items with a short shelf life: The food is usually no longer available for testing by the time the outbreak is known (ex. produce).
  • Spoiled food: A pathogen may be difficult to find if other organisms have overgrown it.
  • Leftover food or food in open containers: The food may have been contaminated after the outbreak or from contact with the food that actually caused the outbreak.
  • No test exists: Some pathogens do not have a test that can detect them in the suspected food.