Historically, modern nursing based on the Nightingale model used disease entities from medical classifications to organize their thinking, speaking, and writing. These were the only concepts available in health care delivery up to the middle of the 2Oth Century. Only recently has there been an interest in the substantive structure of nursing knowledge (Tripp-Reimer, Woodworth, McCloskey, Bulechek, 1996) and, consequently, middle range theory development of diagnostic concepts (Eakes, Burke, and Hainsworth, 1998). With an increase in nursing research there has also been a search for useful classification systems for coding studies (Larson, Dear, and Keitkemper, 1991).

It is well to remember that classification system development parallels knowledge development in a discipline. Even basic structures for knowledge development in nursing had to await the introduction of theories and philosophies of nursing in the 1950s. Today many of the difficulties experienced in classification development are due to, but also are enriched by, the theoretical pluralism in nursing.

National nursing organizations have advanced the idea of classifications for nursing practice. Dr. Gertrude Torres (National League for Nursing) and Dr. Roberta Thiery (American Nurses Association-ANA) were liaisons from two organizations during the early decades of classification work in the United States. ANA has developed criteria for recognizing nursing classifications (Warren, 1997) and has recognized the work of NANDA and the Iowa projects discussed below, as well as the home health care (Saba, 1992) and community classifications (Martin and Sheet, 1992).

This permitted the nomenclatures (diagnostic labels) to be added to the literature search terminology of the National Library of Medicine, the Unified Medical Language System (UMLS). A Unified Nursing Language System (UNLS) "would allow linking or mapping of similar terms while maintaining the integrity and purpose of each classification system. The UNLS is the foundation on which the nursing profession develops, analyzes, and uses national data sets" (Warren, 1997). Hoskins (1997) has reported on the mapping of the aforementioned diagnostic classifications.

Early Classifications

Simply stated, classification is the ordering of entities into groups or classes on the basis of their similarity, minimizing within-group variance and maximizing between-group variance (Bailey, 1994). As nursing knowledge development increased and "entities" (diagnostic categories) were identified, interest in organizing knowledge for practice, education, and research also increased. A very, early reference to building knowledge in nursing is found in Bertha Harmer's Methods and Principles of Teaching and Principles and Practice of Nursing published in 1926 (Aydelotte & Peterson, 1987). She asked

1. whether "organized content of nursing knowledge could be built up as had resulted in medicine and other fields" and
2. "Should nurses not prescribe nursing care for each patient as doctors prescribe medical care?" and
3. she applied the term "social diagnosis" to nursing, a term possibly borrowed from social work (Richmond, 1917).

As nursing theories and nursing process problem-solving were introduced in mid-20th century, concepts of practice emphasized procedures, tasks, and functions. It was in this practice milieu that Abdellah (1959) reported a classification of nursing problems based on a survey of 40 schools of nursing. Consistent with the times, the 21 problems were therapeutic problems that described therapeutic goals of the nurse, rather than health problems of the patient or family. The classification of these goals of nursing served to organize curricula and practice for many years.

A second classification of basic, functional needs was developed by Henderson (1966). The components describe problem-areas; thus it is a conceptual classification into which empirical entities may be classified. At the time no entities, such as nursing diagnoses, existed. These two, early classification systems and the developing theories or philosophies of nursing were influential in setting the stage for the next phase of knowledge development: diagnostic, intervention, and outcome concepts.

The process of classification will be discussed in the context of diagnostic classification. When processes are similar, only differences will be considered in the discussion of intervention and outcome classifications.