Because no unit of measurement is exact, any time you measure something (observed score), you are really measuring two things:
True Score – the amount of the observed score that truly represents what you are intending to measure
Error Component – the amount of other variables that can impact the observed score
IOW: if you weigh yourself today and weigh 140 lbs, and then weigh yourself tomorrow and weigh 142 lbs, is the two pound increase a true measure of your weight gain or could other variables be involved?
Other variables may include: weight of clothing, food intake, placement of the scale (e.g., tile, wood, carpet), error in the scale itself, etc.
