If you’re starting your first research project using retrospective data—like national surveys, electronic health records (EHRs), or claims databases—you’ll want a framework designed for observational research. That’s where PEO comes in.

PEO helps you clearly define your Population, Exposure, and Outcome, so you can ask meaningful research questions using data that already exists.

Not sure how PEO compares to PICO? Check out our PEO vs PICO guide for a full breakdown.

In this article, we’ll walk you through:

• How the PEO framework works
  
• Step-by-step guidance to write your first PEO research question
  
• Practical examples to make it crystal clear
  

Why Use PEO?

PEO is specifically designed for retrospective and observational studies, where you’re examining variables that naturally exist, rather than introducing an intervention or policy.

Using the wrong framework can:

• Make your research question unclear
  
• Complicate data analysis
  
• Reduce clarity when presenting or publishing your results
  

By using PEO, you ensure your research question is focused, actionable, and aligned with the data you have. 

Breaking Down PEO

Population

The population is the group you are studying. Be as specific as possible—consider age, condition, setting, or demographics. 

Good Examples: Residents in internal medicine programs. 

Exposure

The exposure is the naturally occurring factor you want to investigate. This could be a health condition, lifestyle factor, environmental exposure, or social determinant. 

Examples: High workload, smoking status, or low physical activity.

Unlike PICO, which focuses on interventions introduced by a researcher, PEO studies naturally occurring exposures. For a refresher on when to use PICO instead, see our PEO vs PICO guide.

Outcome

The outcome is what you’re measuring or observing in your population. 

Example: Burnout levels, blood pressure, or medication adherence.

PEO in Action: Sample Questions

1. Question 1: In residents of internal medicine programs, are overnight call shifts associated with increased burnout levels?

• Population: Internal medicine residents
  
• Exposure: Overnight call shifts
  
• Outcome: Burnout
  

2. Question 2: Among adults with hypertension, is smoking associated with lower adherence to antihypertensive medications?

• Population: Adults with hypertension
  
• Exposure: Smoking
  
• Outcome: Medication adherence
  

3. Question 3: In elderly patients, is low physical activity associated with higher BMI?

• Population: Elderly patients
  
• Exposure: Low physical activity
  
• Outcome: BMI
  

Notice how these are observational questions—you’re measuring naturally occurring factors, not introducing a new treatment.

Writing Your First PEO Question

1. Define your population: Who are you studying? Be specific, and make sure your dataset includes enough participants.
2. ‍Identify your exposure: What naturally occurring factor are you examining?‍
3. Select your outcome: What measurable result are you observing?‍
4. Check your dataset: Ensure your population and exposure exist in your data and that there’s enough variation for meaningful analysis.
5. ‍Formulate the question: Combine the three elements into a clear sentence (i.e., In [Population], is [Exposure] associated with [Outcome]?)

For a more detailed "how to", check out our step-by-step guide to writing your first PEO research question.

Tips for Success

• PEO doesn’t explicitly list a comparison group, but your analysis inherently compares participants with vs. without the exposure.
  
• Keep your exposure and outcome specific and measurable.
  
• Always confirm your dataset supports the question you want to ask.
  

Next Steps

Once you’ve defined your PEO question, you can:

• Conduct a literature search based on your question
  
• Plan a research timeline
  
• Prepare for data analysis
  

To see more examples and understand when PICO might be the better framework, check out our PEO vs PICO guide.

Ready to put PEO into practice? see our step-by-step guide to writing your first PEO research question.

At Lumono, we help medical trainees like you turn retrospective data into publishable insights. From writing your first PEO question to analyzing your dataset, our tools and guidance support every step of your research journey.