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.
Depression is the most common major mental illness and is characterized by both emotional and physical symptoms. 1  Symptoms of depression are: 2  Intense sadness and despair. 3  Anxiety. 4  Loss of ability to concentrate. 5  Pessimism. 6  Feelings of low self esteem. 7  Insomnia or hypersomnia. 8  Increased or decreased appetite.