Since the 19th century, the scientific method has been applied to investigating, categorizing, and working with groups. In 1986, Dagley, Gazda, and Pistole defined groups as psychologically interdependent and interactive individuals joined in a common goal.

Groups may come together for remedy, prevention, or enrichment through guidance, therapy or psychotherapy. These are some of the most common group classifications by task:

1. Problem-solving groups - although these groups deal with problems that could probably have been solved through individual effort, the group adds diverse resources, error safeguards, and motivation.

2. Education groups - exemplified by study groups and designed to advance learning, education groups should have demographically similar members with different ability levels.

3. Experimental groups - these include T-groups as well as therapy, encounter, personal growth, and sensitivity-training groups. Since the idea is that simply being part of the group will be good for its members, screening is almost always recommended.