Thomas, David A., and Emily Heaphy. "Personality Types: The Myers-Briggs Type Indicator (TN)." Harvard Business School Teaching Note 498-069, March 1998.
Based on the results of MBTI questionnaires, individuals are classified as one of 16 personality types consisting of combinations of the above—e.g., INFP (Introvert, Intuitive, Feeling, Perceiving).
Your MBTI result has four letters, one from each category, so there are a total of 16 different personality types that people could fall under. But to decide if this test was well-made ...