The examiners must be able to determine the effectiveness of the initial testing, and know when retesting is warranted. For example, false positives can occur. This is when people react to the question, but in fact are telling the truth. The skill of the examiner is essential in recognizing the possibility of a false positive, and the need to take additional measures, such as retesting. Polygraph is not a perfect science but, in my opinion, it is a very valuable tool to ferret out those who would commit espionage against the United States.
Confidence starts with estimates of validity.
An estimate of validity can be established from laboratory studies and by projecting the experiences of actual testing. Next, you must estimate the base rate, or the percentage of a population that bears the characteristic we are seeking. Estimates of the number of spies in any organization vary. My estimate in 1994 was there might be a maximum of 3 spies in a population of 10,000. In other words, the error rate on those who pass the test is very miniscule.