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.
Other possible symptoms of psychotic illnesses include: 1  Disorganized or incoherent speech. 2  Confused thinking. 3  Strange, possibly dangerous behavior. 4  Slowed or unusual movements. 5  Loss of interest in personal hygiene. 6  Loss of interest in activities. 7  Problems at school or work and with relationships.