Step 3: Generate Hypotheses about Likely Sources

Illustration representing interviewing people that are sick by showing a telephone

Hypothesis generation is an ongoing process. Possible explanations of an outbreak are continually changed or disproved as more information is gathered. Interviews, questionnaires, and home visits are helpful in narrowing down how and where people in the outbreak got sick.

How are hypotheses generated?

Health officials use three types of data to generate hypotheses about the likely source of the outbreak: epidemiologic, traceback, and food and environmental testing. Investigators begin by trying to pinpoint how the pathogen spread. They review details such as:

  • The specific pathogen causing illness
  • Where sick people live
  • How old they are, their sex, and race/ethnicity
  • Did they have contact with a sick person

Why are interviews conducted?

  • When a contaminated food is suspected, investigators must consider many different foods that may be causing the illness
  • Interviews help to establish a list of foods people ate before getting sick and collects information on other exposures such as restaurants where the ill person ate and stores where they bought food.

This list is used to help investigators determine what food or ingredients the sick people have in common

Key words:

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

Hypothesis-Generating Interviews: Interviews with sick people that disclose what and where they ate in the days or weeks before they got sick

Incubation Period: The time it takes to get sick after eating a contaminated food

Shotgun Questionnaire: A standardized questionnaire with a list of questions about foods, meals, shopping habits, travel, restaurants, and events attended

PHoto of a refrigerator with common graceries found inside including fruits, vegetables, and various meats.

What are hypothesis-generating interviews?

Pathogens that cause foodborne illness can also spread by contaminated water or by direct contact with an ill person or direct or indirect contact with an infected animal. Health officials interview people to find out what and where the ill person ate and went in the days or weeks before they got sick. These interviews are called hypothesis-generating interviews. Depending on what is already known, interviews may focus on specific foods. For example, if several cases are connected to a restaurant, hotel, or event, interviews will focus on the menu items prepared, served, or sold there. The time period investigators ask about depends on the pathogen’s incubation period, which varies for different pathogens.

Visit this page to view sample questionnaires and surveillance tools.

What is a shotgun questionnaire?

If there is no obvious place where ill person got sick (like all eating at the same restaurant), investigators may use a standardized questionnaire known as a shotgun questionnaire. Shotgun questionnaires include questions about:

  • A standard list of food items
  • Each meal a person ate before becoming ill
  • Food shopping habits
  • Travel
  • Restaurant dining
  • Attendance at events where food was served

Shotgun interviews can only suggest hypotheses that are contained on the questionnaire. This approach may not lead to an exact and testable hypothesis. Intensive open-ended interviews can help in this situation.

What do investigators do with information collected from interviews?

  • Create a short list of the foods and drinks that sick people have in common
  • Look at other information, such as past experience with the suspected pathogen, and the age or ethnicities of sick people
  • Make a hypothesis about the likely source of the outbreak

Challenges of Hypothesis Generation

  • Interviews rely on memory. It can be several weeks from the time someone becomes sick to confirming they are part of an outbreak. Someone may not remember in detail what they ate that long ago.
  • The task is even harder when the contaminated food is an ingredient (such as eggs, spices, or herbs) because people often don’t remember or know specific ingredients of the foods they ate.
  • Multiple interviews may be necessary as new ideas arise about possible sources.

These challenges may prevent a hypothesis from quickly appearing. Home visits can be helpful to look at the foods in a sick person’s pantry and refrigerator. With permission, it can also be helpful to review information from their shopper cards.