The amount of stress that you impose on a unit during a pass fail test (probit analysis) or an accelerated life test. For example, you might use a number of stress values to test under common stress loads. If you were testing a drill press that is commonly run at four different RPMs for motor breakdown, you would use each of these different stress levels in your investigation. In cases where the expected life is too long to test practically, you may utilize stress levels higher than what would normally occur in practice to induce early failures. For example, an engine with an average life of many thousands of hours when run under a standard speed may be tested under a stress value of twice this speed. The test can be completed in a fraction of the time, and time to failure at the normal use conditions can be extrapolated from these elevated stress values. Knowledge of the system being tested must be brought to bear when choosing elevated stress levels; stress levels that are too high may break down a system suddenly rather simply accelerating wear. For example, stress testing a paper egg carton's compression strength at several thousand pounds of pressure would cause a nearly immediate breakdown and would offer little insight into the carton's strength under normal conditions.