ISO 3951-4-2011 pdf free download.Sampling procedures for inspection by variables Part 4: Procedures for assessment of declared quality levels.
In any assessment procedure based on sampling, there will be an inherent uncertainty due to possible sampling fluctuations. The procedures in this part of ISO 3951 have been conceived so as to lead to contradiction of the declared quality level only when there is sufficient evidence to support a conclusion that the actual quality is poorer than the declared quality level.
The plans have been devised in such a way that their operating characteristic curves match those of the corresponding attributes plans ISO 2859-4 as closely as possible. Details of the matching method are given in Annex A. The attributes plans of ISO 2859-4 were selected such that when the actual quality level is equal to or better than the declared quality level, the risk is less than 5 % of contradicting the declared value.
It follows that when the actual quality level is worse than the declared quality level, there is a risk that the procedures will fail to contradict an incorrect declared quality level. Owing to the fact that the match between corresponding OC curves in ISO 2859-4 and ISO 3951-4 is imperfect, the corresponding risk in this part of ISO 3951 varies around 5 %. This risk depends on the value of the quality ratio, i.e. the ratio between the actual and the declared quality level. The limiting quality ratio, LQR, is introduced to denote the highest quality ratio considered tolerable. When the actual quality level is LQR times the declared quality level, the procedures in this part of ISO 3951 have a risk of 10 % of failing to contradict the declared quality level (corresponding to a 90 % probability of contradicting the incorrect declared quality level).ISO 3951-4 pdf download.