For example, ecological analyses are often performed on data collected before and after the introduction of a national vaccination program. They can also be performed after a major natural disaster to see if there were any public health consequences.
Ecological analyses are not limited to researching the effects of health interventions. They can also be used to analyze the impact of political or environmental changes and natural disasters on health or to assess non-health outcomes.
The sole defining characteristic of ecological analyses is that the unit being analyzed is the population, not the individual. They are based on population statistics and do not generally take into account the timeline or details of any specific person’s health.
For example, an ecological study that looks at abnormal Pap smear rates before and after the initiation of a nationwide HPV vaccination program would not look at whether any particular individual had been vaccinated. Instead, it would simply look at the prevalence of abnormal results in the years before and after vaccinations had begun.
Examples
Ecological studies have been used to refute the proposed link between autism and the MMR vaccine. When researchers have examined autism rates before and after the initiation of vaccination programs (or before and after changes in vaccine compliance), they have seen no correlation between autism and vaccination.
Instead of a relationship with vaccines, it appears that autism rates have climbed slowly over time—possibly due to changes in diagnostic criteria and/or unidentified environmental factors.
Another example of ecological analysis is an examination of the effect of HPV vaccination on abnormal Pap smears or on cervical cancer rates. Several studies in countries with a far wider uptake of the HPV vaccine than in the United States have done just that.
Research in the United Kingdom and Australia has shown decreases in genital warts, as well as a decline in pre-cancerous cervical changes.
