Public Health: Study Types

• Starts with disease and goes backwards in time looking for the exposure.
• Compares a group with the disease and without the disease.
• The main outcome is odds ratio.
• Better for rare diseases.

Advantages:

• They are fairly quick and cheap.
• Usually are the most important way to investigate rare diseases.
• They don't require a huge number of subjects.
• You investigate multiple causes of a disease.

Disadvantages:

• Often relies on a person remembering things which can lead to recall bias.
• Only investigate cases that have been identified or diagnosed.
• Selecting controls relies on the researcher's judgment.
• Cannot determine the rate or risk of a disease in the exposed and nonexposed.
• Cannot prove a cause and effect.

• Purely descriptive.
• Often describe a new disease with an unclear cause.
• No control group
• Considered the lowest level of evidence, but also the first line of evidence
• Do not address causality
• Do not provide prevalence

• Starts with exposure. 
• Looks at a group over time.
• Grouped together because there is a common experience or exposure.
• Looks for relative risk.
• Looks for exposure and then disease.
• Does not work with rare diseases.

Advantages:

• They are the only study type that can determine the absolute risk of contracting a disease. 

• Survey of a population at a single point in time (usually a day or less than month) -- a "snapshot" in time.
• Used for population- based surveys to determine the prevalence of disease.
• Measures the outcome and the exposure(s) in the population at the same time.
• Frequency of disease and risk factors are identified.
• The main outcome is prevalence.
• Can't determine relative risk or odds ratio.*
• Do not address causality.

* You can estimate OR.

• Patients in a crossover study will "crossover" to another treatment arm during course of the trial.
• This means that even if they are initially put into a placebo arm, they will also eventually receive the study drug or standard of care during the trial.
• Data integrity is ensured by instituting a washout period—a gap in between arms where patients don't receive any medication—in order to reduce carryover effects from the previous treatments.
• Crossover studies are often used when researchers feel it would be difficult to recruit participants willing to risk going without a promising new treatment.

A meta-analysis is a statistical technique that allows researchers to combine the findings of multiple independent studies. It is the type of study that is most frequently used to assess the clinical effectiveness of a healthcare intervention and provides the highest level of evidence of the choices given.

Meta-analysis would be used for the following purposes:

• To establish statistical significance with studies that have conflicting results. 
• To develop a more correct estimate of effect magnitude To provide a more complex analysis of harms, safety data, and benefits.
• To examine subgroups with individual numbers that are not statistically significant.
• Provides the highest level of evidence for efficacy of a drug.

• A study design that randomly assigns participants into an experimental group or a control group.
• As the study is conducted, the only expected difference between the control and experimental groups in a randomized controlled trial (RCT) is the outcome variable being studied.
• You must have an intervention and a comparison group.

* Randomized double blind placebo control studies are the "Gold Standard" research design in intervention based studies.