A study of 100,000 people (reported in the February 2003 journal of Sleep) found that people who reported sleeping eight hours a night had a higher mortality rate than those who reported sleeping seven hours. The longer people slept, the higher the risk of death. What type of study was conducted

The type of study described belongs to the category of Meta-Analysis research, due to it involves analysis over individual studies. In this case, several variables and mortality causes were analyzed, to establish a common element in all patients. Therefore, the relationship between sleep time and cessation of life was made over previous studies.

Explanation

Primary Research: Experimental. Includes experiments on animals and basic elements of human physiology such as cells, genetic structures, and medicines. Clinical. Research to study the effects of medicine on human subjects, and also elements related to how to prescribe this medicine. Epidemiological. Studies frequency and causes of occurrence of diseases, from a historical perspective. Secondary Research: Meta-analysis. Research combining information from different studies. Systematic: theoretical research made on written information.

On the given example, the researches are not applying tests directly to individuals. As mentioned in the description, individuals reported how many hours they slept. This is secondary research. Since the description indicate the only information requested to participants was sleep time, the investigator had to access other information. This information could be medical records or autopsy reports. Based on this data, the researcher can determine who lived and who has died, how many hours they slept, and the causes of end of life.