Abstract: The disadvanges of survival ratio method are theoretically analyzed and the formula which can be used to calculate the failure ratio is improved. The concepts about region mean failure rate are set up and some ralated formulas are developed. The improved survival ratio method is proved to be correct through a practical example, and the reason and advange to replace the reliability with region mean failure rate in the determination of reliable life or time interval between overhaul of products are explained.

Key words: reliability, life, interval between adjacent overhauls, survival model