21. Ecological studies. Immigrant studies

Ecological study
- Also called a correlational study
- A study which aims to identify a correlation between a risk factor or protective factor and a disease
- Use data which is already collected, from
  - Census data
  - Registries
  - Etc.
- Doesn’t use data based on individual cases
- Uses data based on the whole population, like disease prevalence, incidence, mortality
- Focuses on specific problems, like within a region or population
- Advantages
  - Inexpensive
  - Easy to carry out
- Disadvantages
  - Prone to bias and confounding
- Example:
  - Comparing the prevalence of a disease in different regions
  - Finding the association between suicide rate and proportion of Protestant religion in different Prussian provinces
The ecological fallacy
- = incorrectly assuming that an association seen at a populational level also reflects association at an individual level
- Example
  - There is good correlation between salt intake and hypertension when comparing different countries
  - However, correlation on an individual level is difficult to demonstrate
Immigrant studies
- Compares immigrants to other populations, allowing the evaluation of the effect of the environment while the genetic background remains the same
- Compares disease incidence between:
  - A group of migrants to a new country
  - Ethically similar people in their native country
  - The native population of the new country
- Example
  - Cancer rates among Japanese immigrants in the US approached the US cancer rate after 2 generations