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.
There are five major categories of mental illnesses: 1  Anxiety disorders. 2  Mood disorders. 3  Schizophrenia and psychotic disorders. 4  Dementia. 5  Eating disorders.