Random errors occur from unpredictable factors such as fatigue, inattention, mechanical inaccuracy or simple mistakes. They are as likely to increase the observed score as to decrease it. Random errors of measurement are due to chance and can affect a subject's score in an unpredictable way from trial to trial.
Systematic errors are primarily a concern of validity, because, although they are consistent, test values are not true representations of the quantity being measured. By definition, systematic errors are constant and, therefore, do not present a problem for reliability.
We can correct this error by cutting off the extra length at the end of the tape or by subtracting 0.25 in. from the end, measurements of height will consistently record values that are too long by 0.25 in. For example, if the end of a tape measure is incorrectly marked, so that markings actually begin 0.25 in. Therefore, if a systematic error is detected, it is usually a simple matter either to correct it by recalibrating the system or to adjust for it by adding or subtracting the appropriate constant. They occur in one direction, consistently overestimating or underestimating the true score. Systematic errors are predictable errors of measurement. To understand reliability, we must distinguish between two types of measurement errors. Therefore, we must come up with some way of estimating how much of our measurement is attributable to error and how much represents an accurate reading. In reality, we cannot calculate these error components because we do not know what the true score really is. On a second assessment, if we measure 66 in., our measurement error will be +0.5 in. For example, if we measure a height of 65 in., when the true height is 65.5 in., our assessment will be too short that is, our measurement error is −0.5 in.
The difference between the true value and the observed value is measurement error, or "noise" that gets in the way of our finding the true score. The true component is the score the subject would have gotten had the measurement been taken by a perfect measuring instrument under ideal conditions. This expression suggests that for any given measurement ( X), a hypothetically true or fixed value exists ( T), from which the observed score will differ by some unknown amount ( E).
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