For colleagues elsewhere in Europe who are building this for the first time: we have been doing an annual pay survey with our unions in Sweden for a long time, in a forestry and paper company of about 3,900 employees, and a few habits may save you effort.
First, the comparison between different jobs of equal value is where the findings are. Same-job comparisons rarely show much. Comparing a female-dominated group such as payroll and customer service coordinators with a male-dominated group of equal demands, such as maintenance planners, is where we found differences of 6 to 9%.
Second, do it with the union representatives in the room from the first step, not at the end. Third, write the action plan with costs and dates, because next year's survey starts by checking last year's plan.