In a cluster randomized trial each individual in a cluster is randomized to a different treatment.
An ecological study is a particular type of epidemiological study in which one of more potential risk factors associated with environmental issues are related to the outcome of interest with a view to assessing the relationship between the factor(s) and the outcome in individuals.
A multicentre study may be experimental or observational.
Different study protocols are used in two or more of the centres in a multicentre study.
The unit of observation in a medical study is always the individual person.