There is no established optimum percentage of random tests that will ensure the most effective deterrence and detection program. Logically, a very small percentage (e.g., 5 percent) may not be sufficient to deter or detect someone abusing an illicit substance, while a very high percentage (e.g., 95 percent) might be much more than is needed to maintain an effective program. The Department of Transportation permits the percentage of random tests to vary depending on the positive rate. In some cases, an industry is permitted to test at a 10 percent random rate while another industry is required to test at a 50 percent rate. Please contact DOT at 202-366-3784 for additional information on its policy.