Well, to be fair we don't know much Jung had invested in this pop quiz. Or what validation process it has gone through. I did see the chance there is a "fake" scale, though I don't know how that is applied when results are immediately given to the taker.

For it to be valid it has to truly measure the quality intended and do so consistently over time. That doesn't mean the same score, as people change, have moods, don't feel strongly about some questions and answer differently depending upon whether they have had zero or 2 cups of coffee.

The MMPI and it's various iterations have had years and years of work done on it and is generally accepted as valid. I've found it to be uncannily accurate and predictive of future behavior. It has "fake positive" and "fake negative" scales.

Measuring minds and personalities is difficult. Far more difficult than building a rocket, for instance. Rockets follow "hard" science, should behave the same each time following materials science and physics exactly. And yet they blow up from time to time.

It's far more understandable that psychology would have difficulties as it sorts through the wheat and chaff of measuring the human psyche. And yet often the "hard" science people who should not experience failure condemn psychology which has a far more difficult task.

One of Jungs' pithy sayings is "the center of all iniquity is always 5 miles behind enemy lines". We see that played out here every day. Ours is "collateral damage", excusable, unfortunate, but the way it goes on the way to a greater good. We see their actions as "terrorism", cowardly, cruel and contemptible.

I remember taking "Psychological Test Construction" in college, and as part of it worked with 3 other people designing a test to measure introversion v. extroversion. We had a "fake" scale built in. For the folks who took it we labeled it a "Personality Interests Survey". Seemed to be a good test, the professor said it was as good as any on the market.

At the same time I worked for a professor grading papers. He used questions from a bank of they he had saved over the years and added new ones every year. Part of my job was to analyze the questions. I was looking for those which the A students answered correctly significantly more often than the D and Fs and those which they missed at more or less the same rate. We kept the ones, and threw out the others.

Not the best validation techniques, but pretty good for what it took.