Constant voltage methods include one in which the output voltage is regulated to a constant value under all conditions and one in which the output voltage is regulated based on a constant ratio to the measured open circuit voltage ( V O C \displaystyle V_OC ). The latter technique may also be labeled the "open voltage" method.[23] If the output voltage is held constant, there is no attempt to track MPP, so it is not strictly a MPPT technique, though it does function in cases when MPP tracking tends to fail, and thus it is sometimes used supplementally. In the open voltage method, power delivery is momentarily interrupted and the open-circuit voltage with zero current is measured. The controller then resumes operation with the voltage controlled at a fixed ratio, such as 0.76, of the open-circuit voltage V O C \displaystyle V_{OC} .[24] This is usually a value that has been predetermined to be the MPP, either empirically or based on modelling, for expected operating conditions.[19][20] The array's operating point is thus kept near MPP by regulating the array voltage and matching it to the fixed reference voltage V r e f = k V O C \displaystyle V_{ref=kV_{OC}} . The value of V r e f \displaystyle V_{ref} may be chosen to give optimal performance relative to other factors as well as the MPP, but the central idea is that V r e f \displaystyle V_{ref} is determined as a ratio to V O C \displaystyle V_{OC} . One of the inherent approximations in the method is that the ratio of MPP voltage to V O C \displaystyle V_{OC} is only approximately constant, so it leaves room for further possible optimization.