Answer:
A longitudinal study
Explanation:
A longitudinal study can be used to discover relationships between variables that are not related to various background variables. This observational research technique involves studying the same group of individuals over an extended period.
In developed countries, the birth and death rate are low, whilst in developing countries, both the rates are high. Developed countries usually are more industrialized and have higher per capita income levels, whereas developing countries, are less industrialized and have lower per capita income levels.
