Cluster analysis
The regression tree analysis determined four clusters (Figure 4 ) that were significantly different (non-parametric Kruskal-Wallis equality-of-populations rank test; Chi-squared test = 38.7 with 3 d.f. and α = 0.05; P -value = 0.0001). These four clusters were classified as, respectively “low importance” with 22 drivers, “moderate importance” with 14 drivers, high importance and very high importance each with 5 drivers each. The drivers in the “very high importance” node are the five highest ranked, mentioned in the previous paragraph.