Attribute Screens

Attribute screens are screening tests performed on a sample with the results falling into one of two categories, such as acceptable or not acceptable. Because the screen tests are conducted on either the entire population of items or on a significantly large proportion of the population, the screen test must be of a non-destructive nature.

Screening programs have the following characteristics:

1) A clearly defined purpose

2) High sensitivity to the attribute being measured (a low, false negative rate)

3) High specificity to the attribute being measured (a low, false positive rate)

4) Benefits of the program outweigh the costs

5 )Measured attributes identify major problems (serious and common)

6) Results lead to useful actions

Common applications of screening tests occur in reliability assessments and in the medical screening of individuals. In reliability assessments, an attribute screen test may be conducted to separate production units that are susceptible to highs initial failure rates. This period is also known as the infant mortality period. The test simulates a customer use of the unit, or perhaps an accelerated condition of use. The number of failures, per unit of time, is monitored and the screen test continues until the failure rate has reached an acceptable level.

The screen test separates acceptable items from failed items, and an analysis of the failed components is performed to find the cause of the failure. In medical screening, a specific symptom or condition is targeted and members of a defined population are selected for evaluation. Examples of this type of screening include a specific type of cancer or a specific disease. In many cases, the members of the selected population may not be aware that they have the condition being screened. Medical screening tests have the ultimate objective of saving lives.

Near the beginning of the 20th century, Carl Johansson of Sweden, developed steel blocks to an accuracy believed impossible by many others at that time. His objective was to establish a measurement standard that not only would duplicate national standards, but also could be used in any shop. He was able to build gage blocks to an accuracy within a few millionths of an inch. When first introduced, gage blocks or “Jo” blocks as they are popularly known in the shop, were a great novelty. Seldom used for measurements, they were kept locked up and were only brought out to impress visitors. Today gage blocks are used in almost every shop manufacturing a product requiring mechanical inspection.

They are used to set a length dimension for a transfer measurement, and for calibration of a number of other tools. We generally distinguish three basic gage block forms -

• Rectangular

• Square

• Round

The rectangular and square varieties are in much wider use.

Generally, gage blocks are made from high carbon or chromium alloyed steel. Tungsten carbide, chromium carbide, and fused quartz are also used. All gage blocks are manufactured with tight tolerances on flatness, parallelism, and surface smoothness. Gage blocks should always be handled on the non-polished sides. Blocks should be cleaned prior to stacking with filtered kerosene, benzene or carbon tetrachloride. A soft clean cloth or chamois should be used. A light residual oil film must remain on blocksfor wringing purposes.

Block stacks are assembled by a wringing process which attaches the blocks by a combination of molecular attraction and the adhesive effect of a very thin oil film. Air between the block boundaries is squeezedout. Light pressure is used throughout the process.

• The old designation of AAA has a new designation of 0.5 and an accuracy of + or – 0.000001

• The old designation of AA has a new designation of 1 and an accuracy of + or – 0.000002

• The old designation of A+ has a new designationof 2 and an accuracy of + 0.000004 and -0.000002

• The old designation of A and B has a newdesignation of 3 and an accuracy of + 0.000008 and – 0.000004