All the tests at SimilarMinds.com are developed based on scientific measures used to ensure test reliability and validity. Multiple statistical measures and research are used to ensure test questions measure what they are intended to measure. Some of the free tests are experimental versions but even those tests are made up of pre-tested and validated questions.
The field of personality testing dates back to the 19th century when Sir Francis Galton created the first psychological questionnaire and Alfred Binet created the IQ test. The field of personality testing as we know it today was trail blazed by Hans Eysenck and Raymond B. Cattell who theorized that personality was made up of behavioral variables called traits or factors. All modern scientifically constructed tests are built on a trait measurement system. The Myers-Briggs test for example measures 4 traits, Big 5 - five traits, etc.. The Enneagram which was not scientifically developed has nine traits.
My research work is focused on exploring personality models and test structure. Since I focus more on test development and data analysis, make sure you search elsewhere on the web for longer descriptions of the test results I give here. Be mindful that many of the writings on personality are based on non scientific observations so they will contain both intuitive hits and misses. The field of personality testing is still immature so it is wise to be skeptical of your results on any test. Furthermore, until researchers are successful finding and proving a global personality model, I think there are questions as to what many of the traits currently tested for (on the various personality systems) mean in the big picture.