When an attempt is made to arrive at the value of a variate, the result may be exactly correct or it may be, to a certain extent, off the mark. The


of such an evaluation refers to the closeness of the measure or otherwise assessed value to the true value. The closer the assessed value is to the true value, the more accurate the evaluation. If for some reason the assessed values tend to be consistently too high or too low, they are


. Bias can be introduced into the evaluation process through faults in the instrumentation, improper techniques, or human error. The presence of bias is often difficult or impossible to detect, and consequently the accuracy of a set of observations is difficult or impossible to ascertain.