Human errors in work systems have occasionally a great effect on quality, safety and efficency. In order to avoid the trubles due to human errors, "fool proof" methods are adapted in various manufacturing processes. Though many factories apply the fool proof methods to their work systems, there is no general methodology to apply them.Ordinaly, the fool proof methods are derived at need in each occasion. In this article, general principles of fool proof methods are extracted from about 700 examples which are collected at assembly lines of mechanical products. As a result, 5 fundamental principles for fool proof methods-elimination, substitution, facilitation, detection of errors and relaxation of effects-are obtained. The first three are the methods fos prevention of human error occurances. The latter two are the methods for suppression of the effect of errors.
This paper deals with the estimation method of the lifetime of a product in which the real operating time is different from its actual calender operating time. During the warranty period of a product, the manufacturer will only collect failure data from the repair requests by the owners. The manufacturer cannot usually obtain the real operating time of non-failure products.Some manufacturers have data about the usage time of the product during the warranty period : for example, mileage that the automobile will have run during that warranty period. In general these data consist of information about both failure and non-failure products. Two nonparametric MLE's about the reliability function of the product are presented. Estimation (A) uses information taken from both failure and non-failure products. Estimation (B) does not distinguish between them. The precision of these two estimators is compared using their asymptotic variances. Estimation (A) proves to have better precision than estimation (B).
Through the study on quality theory in the literatures of philosophy, quality control, the study of merchandise and so on, we made clear that there are two aspects of quality such as subjective and objective ones and that it is necessary to investigate the correspondence of these two aspects. Then, for this purpose, we propose that two dimensional recognition should replace one-dimensional one which has been so far prevailing, and that this recognition of the correspondence is utilized for categorizing quality elements of a product into attractive, nust-be, one-dimensional quality ones and so on.Then the practical validity of this theory is examined through the questionnaire survey about the TV set and the table clock to consumers and it is investigated how each quality element of these items is evaluated under the theory. Moreover, an example of new clock planning with applying this theory is discussed in order to show the practical effetiveness of this theory.