11.Error : a deviation of the measured value from the true value. Errors are divided into three broad classes-gross, systemic, and random. The gross errors are mostly human errors such as incorrect reading, adjustment, and application of instruments. Loading effects , due to improper circuit impedance, also give this type of error. Systemic errors are related to the functioning of the instruments and their mechanical or electrical structure and calibration errors, and to the effect of environment on the equipment performance. These errors related to the instrument can be avoided or reduced by proper calibration of the instrument, and by selecting a suitable instrument . Environmental errors are reduced by using the instrument at the recommended conditions of temperature, relative humidity, pressure, etc . Random errors are due to unknown causes. They can be reduced by proper design of the instrument, and by taking more readings and using statistical methods to correct the observations