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Philip L. Bewig


You know it's true: Spreadsheets have errors like dogs have fleas. It is generally accepted that nine out of every ten spreadsheets suffer some error, and consequences can be severe:

  • A cut-and-paste error cost TransAlta $24 million when it underbid an electricity-supply contract.
  • A missing minus sign caused Fidelity’s Magellan Fund to overstate projected earnings by $2.6 billion (yes, billion) and miss a promised dividend.
  • Falsely-linked spreadsheets permitted fraud totaling $700 million at the Allied Irish Bank.
  • Voting officials reported spreadsheet irregularities in New Mexico and South Africa.
  • A new drug introduction was delayed several months by an untested macro, costing the pharmaceutical company profits and its patients misery.


Traffic light testing
Traffic light testing

Have a colleague check your work before you publish it.

He should review your documentation, analyze your design, critique your test plan, and test for reasonableness.

Cell-by-cell inspection is tedious, but is the only method of spreadsheet testing known to be effective.

A traffic light tool, such as shown in the figure, can help:

  • The auditor has already checked green cells, finding no errors.
  • Yellow cells are ready to check because, as the arrows show, all their precedents are green.
  • Red cells are not ready to check because they have unchecked precedents.


2005, EuSpRIG

Full article

How do you know your spreadsheet is right?