scope:

Chapter I of this handbook provides definitions of terms required for the use of the life test sampling plans and procedures of chapter 2.

Life-test is the process of placing the "unit of product" under a specified set of test conditions and ensuring the time it takes till failure.

The unit of product is the entity of product which may be placed on life test.

The state that constitutes a failure must be specified in advance of the life test.

A life test sampling plan is a procedure which specifies the number of units of product from a lot which are to be tested, and the criterion for determining acceptability of the lot.

Life test sampling plans whereby testing is terminated when a preassigned termination number of failures, r, occur are given in section 2B of this handbook.

Life test sampling plans whereby testing is terminated when a preassigned termination time, T, is reached are given in section 2C of this handbook.

Sequential life test is a life test sampling plan whereby neither the number of failures nor the time required to reach a decision are fixed in advance but instead decisions depend on the accumulated results of the life test. Information on the observed time to failure are accumulated over time and the results at any time determine the choice of one among three possible decisions: (1) the lot meets the acceptability criterion, (2) the lot does not meet the acceptability criterion, or (3) the evidence is insufficient for either decision (1) or (2) and the test must continue. Sequential life test sampling plans are given in section 2D of this handbook and have the advantage over the life test sampling plans mentioned in paragraphs 1A1.6 and 1A1.7 in that, for the same operating characteristic curve, the expected waiting time and the expected number of failures required to reach a decision as to lot acceptability are less for the sequential life tests.

The number of failures required for decision is the number of failures that have occurred at the time the decision as to lot acceptability is reached. For the life test sampling plans mentioned in paragraph 1A1.6, this number of failures is known in advance of the life test; but, for the sampling plans mentioned in paragraphs 1A1.7 and 1A1.8, this number cannot be predetermined. The expected number of failures required for decision is the average of the number of failures required for decision when life tests are conducted on a large number of samples drawn at random from the same exponential distribution. The expected number of failures can be predetermined for the sampling plans mentioned in paragraphs 1A1.6, 1A1.7, and 1A1.8.

The waiting time required for decision is the time elapsed from the start of the life test to the time decision is reached as to lot acceptability. The waiting time required for decision cannot be predetermined for any of the sampling plans mentioned in paragraph 1A1.6, 1A1.7, or 1A1.8. The expected waiting time required for decision is the average of the waiting times required for decision when life tests are conducted on a large number of samples drawn at random from the same exponential distribution. The expected waiting time can be predetermined for the sampling plans mentioned in paragraphs 1A1.6, 1A1.7, and 1A1.8.