Several recent studies show that market concentration in the US has increased over time, with firm profits increasing in the same period. The consistency of findings from the US is contrasted by more varying results from studies of the development of market concentration in Europe. In this study, we utilise the completeness of Norwegian microdata to investigate how methodological choices and data limitations impact results with respect to the market concentration and its relationship with profitability. First, we find that concentration in Norway has decreased slightly over the last two decades. Over the same period, profitability has increased slightly for two profitability measures and been stable for the other two. Despite a difference in overall trends, at the industry level, we find a positive and statistically significant relationship between concentration and profitability for three out of four profitability measures, in line with the market power hypothesis. Investigating the effect of methodological choices and data limitations, we find that concentration trends are quite robust to exclusion of smaller companies, the incorporation of ownership structures in concentration measures and the choice of industry classification. However, the positive relationship between concentration and profitability is almost non-existent when using readily available industry classification instead of more product market-oriented industry classifications and disappears completely when we do not exclude export-oriented industries. Our study is relevant for future research, as well as for policymakers, as our results indicate that one should be careful when interpreting results from studies of market concentration that fail to handle these methodological challenges.