(a) This section contains the Operating Characteristic (OC) curve for each of the sampling plans given in Tables I, I-A, II, II-A, III, and III-A. The OC curve and the corresponding sampling plans are listed by AQL.

(b) Different acceptance and rejection criteria are provided for each AQL. The criteria for each AQL must be obtained from the applicable sampling plan tables.

(c) The curves show the ability of the various sampling plans to distinguish between good and bad lots. This can be illustrated by examining OC curve 6 for an AQL of 0.25 defects per hundred units in the Reduced and Normal Inspection Plans. If the quality of the lots submitted for inspection is poorer than the AQL of 0.25 defects per hundred units, fewer lots will be accepted. For example, OC curve 6 shows that when the quality of lots submitted for inspection is 1.0 defects per hundred units, only 26 percent of the lots are expected to be accepted. Conversely when the quality of the lots submitted for inspection is better than the AQL of 0.25 defects per hundred units, most lots are expected to be accepted. For example, the same OC curve 6 shows that when the quality of lots submitted for inspection is 0.10 defects per hundred units, about 99 percent of the lots are expected to be accepted.

(d) The table of sampling plans that correspond to OC curve 6 can be found over the curves for an AQL of 0.25 defects per hundred units in the Reduced and Normal Inspection Plan. An examination of this table reveals that there is one single and one double sampling plan that have OC curves comparable to OC curve 6. The first plan listed is a single plan requiring the inspection of 500 individual containers. Under this plan the lot is accepted as meeting the requirements for an AQL of 0.25 if there are 3 or less defects in the sample or rejected if there are 4 or more defects in the sample.

(e) The next plan that is listed in the column headed 6 for an AQL of 0.25 is a double sampling plan that requires the initial inspection of 228 individual containers. The lot will be accepted as meeting the requirements of an AQL of 0.25 if there are no defects in the sample, and rejected if there are 3 or more defects in the sample. In the event that the number of defects is between the acceptance (0) and rejection (3) numbers, additional containers must be inspected. In this case, the table indicates that a total of 516 containers must be inspected before a decision can be made to either accept or reject the lot. This will require the inspection of 288 more containers (516 − 228 = 288).

If there are 3 or less defects in the total sample, the lot will be accepted. If there are 4 or more defects in the total sample, the lot will be rejected. The other double sampling plans operate in a similar manner with the only differences being the sample sizes and acceptance and rejection numbers. [31 FR 4687, Mar. 19, 1966; 31 FR 4949, Mar. 25, 1966, as amended at 36 FR 18457, Sept. 15, 1971. Redesignated at 41 FR 42640, Sept. 28, 1976 and 42 FR 32514, June 27, 1977, further redesignated at 45 FR 69424, Oct. 21, 1980 and 46 FR 63203, Dec. 31, 1981]

The Operating Characteristic (OC) curve information (probability of acceptance) for skip lot sampling and inspection procedures described in § 42.121 is easily obtained from the single curve in § 42.142. The procedure for using the curve in § 42.142 is as follows:

(a) Select any sampling plan for normal condition of container inspection from Table I or Table I-A of § 42.109.

(b) For a given Quality of Submitted Lots, 100p, find the Percent of Lots Expected to be Accepted, Pa from the respective OC curve in § 42.140.

(c) Refer to § 42.142 with this Percent of Lots Expected to be Accepted, Pa, and read the new Percent of Lots Expected to be Accepted, Pas, which results when using these skip lot procedures.

For example, let's assume the lot size is 6,001 to 12,000 containers, and we use the single sampling plan of size 168 for normal inspection (Table I of § 42.109) along with an AQL for total defects of 6.5. The OC curve for this sampling plan is curve N and R3. Also, assume that the quality of the lot is 6.5 defects per hundred units. From curve N and R3, then, the Pa is determined to be 95 percent. Then refer to the graph in § 42.142 and locate Pa of 95 percent along the horizontal axis. From this point, proceed vertically to the curve and then horizontally to the left to the vertical axis. From this point on the vertical axis, Pas is determined to be approximately 98 1 percent for skip lot sampling and inspection.

1 This slight increase of Pas over Pa results because all the lots offered for inspection are no longer being inspected. Some lots are being “skipped”, and thereby accepted, which would have been rejected if inspected. The largest increase in Pa which will occur using skip lot sampling is approximately 4 percent. It is for this reason that skip lot sampling procedures are applied only in instances where past production quality has been consistently high—as evidenced by the requirement to have 10 consecutively acceptable lots prior to a reduction in sampling rate—and where all requirements in § 42.120 also have been met.

[41 FR 42641, Sept. 28, 1976. Redesignated at 42 FR 32514, June 27, 1977 and 45 FR 69424, Oct. 21, 1980 and further redesignated at 46 FR 63203, Dec. 31, 1981]

Notes:

1. This curve applies only to the specific skip lot sampling and inspection plan described in §§ 42.121 and 42.123.

2. Pa and Pas are essentially equal for values under 50 percent.

[41 FR 42641, Sept. 28, 1976. Redesignated at 42 FR 32514, June 27, 1977 and further redesignated at 45 FR 69424, Oct. 21, 1980 and 46 FR 63203, Dec. 31, 1981]

(a) This section contains the Operating Characteristic (OC) curve for each of the on-line cumulative sum sampling plans provided in subpart D. The OC curve and the corresponding cumulative sum sampling plans are listed by AQL.

(b) Different plan parameters used to make acceptance and rejection decisions are provided for each AQL.

(c) The curves show the ability of the various cumulative sum sampling plans to distinguish between good and bad portions of production. The interpretation of these curves for portions of production is similar to the interpretation of the OC curves for stationary lots as illustrated in § 42.140(c).

[45 FR 69424; Oct. 21, 1980. Redesignated at 46 FR 63203, Dec. 31, 1981]