The observability is a measure of the ease (or difficulty) with which one can determine the signal value at any logic node in the circuit by controlling its primary input and observing the primary output. Here the term primary refers to the I/O boundary of the circuit under test This measure is of importance because when we desires to measure the output of a gate in a larger system to check it operates properly. Given a limited number of nodes that are directly observed, it is the aim of well-designed chips to have easily observed gate outputs and the to get some basic test design techniques which required in the project. Further, one should be able to observe directly in direction every gate output within an integrated circuit. While at one time this aim was hindered by limited gate-count processes and a lack of design methodology, current design practices and processes allow one to approach this ideal. The observability is simply a function of controllability, meaning that it is impossible to observe a given internal node if the circuit is not driven to a given state.