It refers to the extent to which a measurement scale produces consistent results if repeated measurements are made. Evaluating the reliability of any measuring instrument consists of determining how much of the variation in scores is due to inconsistencies in measurement. The reliability of an instrument should be established before it is used for a substantive research study. In case of reliability it has been found that systematic sources of error do not have an adverse impact because they affect the measurement in a consistent way and do not lead to inconsistency. In contrast, random error produces inconsistency, leading to lower reliability. Therefore, more precisely reliability may be defined as the extent to which a measurement instrument is free from random error.