Studies are conducted using varying methods and are commonly classified as observational or interventional. Common study classifications include:

• Observational studies meaning that researchers only observe study participants and do not intervene in their care or treatment program. These studies are not considered clinical trials.
• Observational retrospective cohort studies collect past data and information from the medical records of two groups of similar human populations who differ in only one key characteristic, to measure a specific outcome. An example of this would be looking for the prevalence of lung cancer in a population of 40-year-old Caucasian women where half of the population smokes tobacco and half do not smoke at all.
• Prospective cohort studies follow a similar human population over a certain time period where the population has key differentiating factors with respect to the study. Researchers observe these differentiations to determine how they affect the study's outcome.
• Registry studies observe and collect clinical data about the effectiveness and use of a medication or medical device in a human population that has already received the medication or device. These studies are typically broad and do not exclude participants based on age, gender, race, etc.
• Case control studies compare a population with a certain medical condition to a very similar population without the medical condition, to determine factors that may lead to the condition.
• Interventional studies mean that researchers give study participants a particular medicine or medical device to assess the outcome. Most clinical trials are classified as interventional studies.
• First-in-man studies are when a medication or device that has previously been tested on a non-living organism or animal is tested on humans for the first time.