Why do psychometric tests not provide perfectly reliable measurements of psychological characteristics?

Answer 1

The fundamental reasons are that psychology, and by association, psychometrics, is a) an incomplete discipline (does not adequately quantify the spectrum of human behavior), b) measurements (typically through question and answer) are interpretive, not quantitative, and therefore subject to error, and c) an individual's behavior may be change in a different context and as such the measurement may not apply ten days later, or even in the moment the measurement is rendered.

To use a physical analogy, imagine a psychologist trying to throw darts at a dartboard to score a bullseye as being representative of his psychometric test. Should be easy, right? (that is to say, if a pshychologist spent his four years of graduate school throwing darts instead he'd probably hit the bullseye a lot). Now imagine the dartboard, instead of being circular with concentric rings and a bullseye in the center, is warped and twisted with maybe a few bullseyes, some larger, some smaller, some further away, some closer, and maybe some even wrapped around the back side of the dartboard so the psychologist can't hit it with a direct throw. To make matters worse, he'll find that other dartboards have general consistencies between them, but in many cases the warps and twists and bullseyes are disturbingly different. His toolset (darts and a throwing arm) will be able to hit some of the targets, but not all, and not on all of the boards. This is sort of the physical analogy of case a).

Now let's suppose the psychologist is drunk, or hopped up on crystal meth, or his arm is in a cast, or he's thinking about that girl who just rejected him at the bar, or maybe he's perfectly fine, just a little nervous about hitting the target and it's taking the edge off his game. His own influences and biases will affect the outcome of his result, his ability to hit the target. This is sort of the physical analogy of case b).

Finally, let's suppose the dartboard, instead of being stationary, flows like blobs in a Lava Lamp so the bullseyes change size, shape and distance slowly over time. That's the human's psyche changing over time, and is the physical analogy of case c).

A better question might be: Why do psychometric tests not provide "better-than-adequate" measurements of psychological characteristics? :-)

Answer 2

Probably because there is always uncertainty with human beings. You would probably get better answers in psychology.

Answer 3

It would be amazing if there were one, but unfortunately, there are none.