REFA, the Association for Work Design, Business Organization and Business Development is Germany’s oldest organization for work design, business organization and business development. They offer training similar to Six Sigma or Lean Management.
If you like to perform a relatively accurate analysis of one month’s hotline calls, you can apply this approach:
Named ranges and formulas:
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Time_Class_Estimate.xlsx [17 KB Excel file, open and use at your own risk]