Stratified sampling is often used when one or more of the stratums in the population have a low incidence relative to the other stratums. Convenience sampling is used in exploratory research where the researcher is interested in getting an inexpensive approximation of the truth.
Sometimes symptoms of a mental health disorder appear as physical problems, such as stomach pain, back pain, headache, or other unexplained aches and pains.
1 Some of the most common types of mental illness include anxiety, depressive, behavioral, and substance-abuse disorders. 2  There is no single cause for mental illness. 3  Rather, it is the result of a complex group of genetic, psychological, and environmental factors.