Meyer, Bernd, et al. “Tertiarisation Without a Competitive Industrial Base? An Empirical Analysis of Shifts in the Sectoral Structure of Employment in the münsterland and in the Emscher-Lippe Region”. Raumforschung Und Raumordnung | Spatial Research and Planning, vol. 57, no. 5-6, Sept. 1999, pp. 386–397, https://doi.org/10.1007/BF03184507.