Where did the instrument come from?
Jung's observations on Personality Type are based on decades of personal and professional experience as a practicing psychotherapist and analyst. When his book Psychological Types was translated into English in 1923, Katharine Cook Briggs and Isabel Briggs Myers were so fascinated that they started their own research, looking at how their friends and acquaintances behaved and noticing the differences and similarities that Jung described.
Most of all, they wanted this knowledge to become accessible to the everyday person dealing with everyday issues. The first major application they used Type knowledge for was helping women figure out which job would best fit their disposition during the war efforts of 1939 and beyond. Since Myers and Briggs could not find a test or indicator of how to quickly discern and figure out which preferences one had, they created their own. From scratch. Peter Myers, Isabel's son, goes into a little more detail in the preface of their book, Gifts Differing and I've written a little more about the instrument here.
Many people mistrust Type tools, either because they were mis-typed in the past, or because they think the result - any result - belittles their adaptability.
Here's my point-of-view:
1. I took a test online and it's wrong.
There are many "personality tests" available online for free. Many even if give you a similar four-letter result. However, those knock-off versions are less likely to yield an accurate profile for two reasons:
1) those "tests" have not undergone the decades of research and polishing of the questions that the MBTI(r) questionnaire has, and 2) it takes a professional debrief to ensure your answers come from the right kind of understanding, e.g. not thinking who you would like to be after more training, or who your parents always told you to be. (Here's a search-tool for Certified Practitioners in your area.)
You are complex.
Your psychological type is more than just four letters and choices between two dichotomies. Your type is dynamic, there is a hierarchy to your functions, and the patterns described by your whole type are so much richer than only looking at what the four letters mean.
Don't waste your time on a free "test", invest in the real thing and have something solid to work with.
2. I don't want to be put in a box
Everyone always has access to all functions and attitudes.
That means there is no "putting in a box", only an awareness of what comes more naturally, and what needs more conscious effort.
The MBTI® indicator helps people identify their innate preferences. Those preferences show how we prefer to use our brain and how we direct our mental energy. While its validity implies that people with similar preferences may behave in similar ways, no two individuals will ever be completely the same, and no behavior will be totally predictable. Type does not explain every last detail: your experiences, upbringing, and culture also have a tremendous influence on how you use your gifts.
3. My Type has changed.
No, it hasn't. Your Type is the starting point, and by definition, starting points don't change. What's happening as you grow older is called Type Development: you have an inner drive to find balance and wholeness, and your experiences over time allow you to flex into the opposite of your preferences more easily.
Type knowledge shows us how to recognize and respectfully bridge our differences. Once we know why we behave the way we do, it is much easier to allow others to be themselves also.
Katharine and Isabel never actually met with Jung, but they did exchange some letters. As I've touched on in Type Theory, it is C G Jung's Personality Type Theory that provides the framework and basis for the Myers-Briggs tool.
Complementing the three dimensions of energy source (Extraversion 'E' or Introversion 'I'), information processing (Sensation 'S' or Intuition 'N'), and decision-making (Thinking 'T' or Feeling 'F') that C G Jung had identified, Katherine and Isabel added another dimension to make up the MBTI®'s four-letter result. This dimension looks at which function people show in the outside world, calling it Judging 'J' or Perceiving 'P'.
During the first three decades since its beginnings in the 1940s, the MBTI® questionnaire was only available for research purposes. It underwent revisions and rigorous testing, proving it to be a statistically reliable and valid instrument. In the 1970s general practitioners were able to get certified and apply it in their practice, and today, over 2 million users trust the MBTI® questionnaire every year in the USA alone.
The MBTI(r) result does not guarantee we are any good at using, for example, our Thinking or Intuition functions, it just shows if we prefer them over the Feeling or Sensing alternatives. Therefore, it is not a suitable tool for recruitment or match-making.