Psychiatric epidemiology
Psychiatric Epidemiology is a field which seeks to measure the prevalence of mental illness in society.  It is a subfield of the more general epidemiology.  It is very difficult to accurately measure such a thing as mental illness prevalence, and current techniques are relatively poor.  Two areas of concern, sometimes called the crisis of psychiatric epidemiology today, are the high estimates of mental illness that many studies produce and the difference in results between studies.

# How Studies are Conducted
The common technique for psychiatric epidemiological research today is structured interviewing, a technique in which a series of questions is administered by lay interviewers to determine whether an individual is disordered or nondisordered.

# Concerns of Epidemiological Research

## Clinical versus Actual Prevalence
Clinical prevalence is a measure of how many people who seek out mental health services actually suffer from mental illness, whereas actual prevalence is a measure of the percentage of people who suffer from mental illness in all of society.  It is much simpler to study clinical than actual prevalence, but the actual prevalence is the area in which epidemiologists are most interested.  Thus, efforts continue to be made to improve research techniques to better estimate the actual prevalence of mental illness.

## Sensitivity and Specificity
Another concern of psychiatric epidemiological studies are the issues of sensitivity and specificity.  Sensitivity is a term which regards whether a given selection criteria detects all of the people with a given mental illness, whereas specificity refers to whether the technique only detects the people with mental illness.

## Validity and Reliability
In order for a study to be taken seriously, it must be both valid and reliable.  Validity refers to whether the study actually measures what it is meant to measure.  Reliability refers to whether the measurement is consistently made in the same way over and over again.  If an experiment is not reliable or valid, it will not withstand the tests of time and peer review.

# Example: The Epidemiological Catchment Area Project
In an attempt to measure the prevalence of mental illness in the United States, Lee N. Robins and Darrel A. Regier conducted a study called the Epidemiological Catchment Area Project which surveyed samples of the general population at five sites across America.
In the study, it was found that about a third of all Americans suffer from mental illness at some point in their lives.  This statistic is often referred to as lifetime prevalence.  Although this seems a sound estimate, it is clear from the criteria used in the study that at some times the specificity and sensitivity of the selection criteria leaves something to be desired.

# Sociological Concerns
There is a negative consequence to the conduction of studies that are not valid and reliable.  The medicalization of symptoms and character traits can have a damaging effect on those that display these symptoms and traits.  For example, a person being surveyed for major depressive disorder in the ECA project would be asked questions such as "In your lifetime, have you ever had two weeks or more during which you felt sad, blue, depressed, or when you lost all interest and pleasure in things that you usually cared about?"  Although many people would answer yes to this question, it is not necessarily because they were experiencing depression.  Rather, they could be experiencing a natural reaction to some kind of life stress.  By labeling this reaction as a mental illness, it increases the stigma of the person's symptoms and thus creates problems for their recovery.
Studies like the ECA do not draw the line between mental illness and normal human stress responses.  This creates many problems for those who suffer from both mental illness and those who don't by decreasing the seeming seriousness of mental conditions and increasing the stigma on those who are suffering a stress response.