The compliance method is based on simple force-displacement data and is successfully applied to determine fatigue crack propagation in linear elastic, isotropic materials like metals and ceramics. Here, we investigate its potential use in non-linear, time dependent materials like polymers, by comparing its results with those of direct optical tracking experiments. The non-linear and viscoelastic behaviour of polymers proves to cause a strong loading condition- and time dependency of the calibration curves and, as a result, no unique relation can be found for crack length as function of dynamic compliance. Normalization of the dynamic compliance, using an apparent modulus, slightly reduces the dierence, but this still does not yield a unique functional description, since the deviations between calibration curves appear to be related to stress enhanced physical ageing during the experiment. Determination of the crack length via optical tracking prevails. When impractical and therefore the compliance method is used instead, results should be taken with care.