Table 3. Non-parametric correlation between a variable related to the surgical procedure (histologically examined nodes or lymphadenectomy) and a variable related to pN category (positive nodes, pN according to the UICC classifications of 1987 and 1997). In the first column, showing Kendall's rank-order correlation coefficient T, the correlation was evaluated without controlling for any other variable. In the second column, showing Kendall's partial rank-order correlation coefficients, the analysis was repeated controlling for a marker of disease progression (depth of tumour invasion).