Input quantities are classified into three categories: i. Desired input ii. Interfering input iii. Modifying Input

Desired Inputs: It is defined as quantities for which instrument or the measurement system is specifically designed to measure and respond. The desired input rD produces an output component cD = GDrD in accordance with an input output relationship.

Interfering Inputs: It represents the quantities to which an instrument or a measurement system becomes unintentionally sensitive. The measurement systems are not desired to respond to interfering inputs on account of their principle of working, design and many other factors. The interfering input rI is operated upon by a transfer function GI to produce an output in the same manner as a desired input is operated upon by a transfer function, GD, to produce an output.

Modifying Inputs: Modifying inputs are defined as inputs which cause a change in input-output relationships for either desired inputs or interfering inputs or for both. Thus, a modifying input, rM, is an input that modifies GD and/or GI. The symbols GMD and GMI represent the specific manner in which rM affects GD and GI respectively. These symbols, GMD and GMI are interpreted in the same general way as GD and GI are.