Determining a measuring uncertainty and/or maximum measuring error of a polarization dependent loss--PDL--tester for determining a PDL value of a device under test--DUT--is provided by using the PDL tester for determining a value of PDL of a verification element having an actual value of PDL greater than a maximum value of a specified measuring range, wherein the PDL tester has an expected measuring uncertainty and/or expected maximum measuring error. The measuring uncertainty and/or maximum measuring error or the tester is then derived from the determined value of PDL of the verification element in conjunction with the actual value of PDL of the verification element.