Introduction

Abstract

The variation among physical observations is a common characteristic of all scientific measurements. This property of observations, that is, their failure to reproduce themselves exactly, arises from the necessity of taking the observations under different conditions. Thus, in a given experiment, readings may have to be taken by different persons at different periods of time or under different operating or experimental conditions. For example, there may be a large number of external conditions over which the experimenter has no control. Many of these uncontrolled external conditions may not affect the results of the experiment to any significant degree. However, some of them may change the outcome of the experiment appreciably. Such external conditions are commonly known as the factors.