The validity of the Myers-Brigg Type Indicator (MBTI), as with every research tool, is established in four phases.
First, there has to be a philosophical or theoretical foundation upon which the research for reliability will be based. Without theory there is seldom a rationale for research. In the case of MBTI, Katharine Cook Briggs and her daughter Isabelle Briggs-Meyer had a goal: to find a way to match people to jobs based on their personality types. As their rationale, Myers and Briggs used the theory of typology as defined by Carl Jung in his book Personality Types. Based on this theory, individuals can be categorized by salient traits which make them better suited for specific kinds of tasks.
The second phase of validity is to extrapolate specific elements from the theoretical foundation and test them within set parameters depending on what is to be tested and for what reason. Myers-Briggs listed specific personality types based on Jungian theory:
Then they obtained a list of wartime jobs during World War II. Their goal was to find a way to match people to suitable jobs for best productivity during these trying times.
The third phase is the actual creation of the assessment tool, which Myers-Briggs built as an indicator. Once created, the indicator would be administered with exact methodology in a wide range of participants to ensure enough data collection and to establish patterns, trends and correlations between jobs and personality.
Finally, the fourth phase would be the data analysis which will reveal whether there are any trends, patterns and correlations. Myers and Briggs succeeded at making a correlation between job types and types of individuals, for which the indicator obtained its actual validity. Moreover, the test proved to be reliable enough to the point of being currently used in over 2 million people every year to test for job aptitude. The more a tool is used and analyzed the reliability and validity of the tool will be more often be reviewed and put to productive usage.
Jung's typology, which is the foundation of the Myers-Briggs indicator, in itself is just is a theory based Jung's (temporary) embrace of Freud's theory, and based on Jung's personal experiences and empirical research. There is no scientific basis for Jung's typology, nor for Freud's theory either. It is theory what actually motivates the extrapolation of factors, phenomena, and facts for the purpose of further research that could result in the discovery of scientific foundations.
Therefore, keep in mind that philosophy, theory, and thesis are just as necessary to experimentation as the actual data and assessment tools. It is all a composite that leads to establishing a quantitative or qualitative connection.