Slug flow is a common flow pattern, which is often accompanied by undesired effects, like pressure loss or vibrations, leading to large errors in multiphase flow metering. Because these undesired effects strongly correlate with the frequency of slug occurrence, this parameter is of special interest. In this paper, different slug frequency calculation methods are applied to data from multiphase flow simulations and corresponding high-speed video observations for six test cases within the plug / slug flow regime. Commonly used methods, like power spectral density (PSD) or calculating the mean slug frequency by applying a fixed threshold, are compared with new evaluation methods. Since every approach has its pros and cons, it is recommended to apply different methods to each data set. The deviations in the resulting slug frequencies indicate how much one can trust the results. If large variations are observed, one should apply an advanced technique for the calculation of the liquid level / hold-up, which takes aeration into account.