The dispersity of 50% discharge voltage U50 and standard deviation a achieved from the multistage method for impulse voltage test is analyzed. The verification with computer simulation technique shows that the deduced formulae for calculating σU50 andσσare reliable.For improving the accuracy of the achieved results some rules for using the multistage method based nothese formulae are demonstrated. The voltage stages should be equal to or greater than 5. According to the value of σ/U50 the number of shots in each voltage stage should be 10

20 or 40.