The procurement and maintenance of equipments are important for higher productivity. An equipment can have several grades according to its performance or efficiency, thus we judge the grade of the equipment to be low to high. Sometimes, the judgement can not be correct. This paper presents the inference about the posterior probability that an equipment is in one grade under the condition that it was previously judged to be in another grade, and gives an related example. Using this inference idea, productivity enhancement can be possible through the concept of more exact judgement on better equipments.