Minimize Error Variance:
A third group of factors influencing the dependent variable are known as random variables because they are temporary factors which vary from situation to situation. Their influence is considered as error variance. There are a number of determinants of error variance for instance factors associated with individual differences among subject due to errors of measurement variation of response from one trial to another guessing momentary in attention temporary fatigue lapses of memory and transient emotional state of subjects. Minimization of error variance can be achieved through:
a. Measuring under controlled conditions and
b. By increasing the reliability of measure or measuring instruments.