Many studies over the past ten years have shown that up to 90% of models contain significant errors. Generally these errors go undetected, but there have been some well-publicised cases where they have resulted in financial loss and embarrassment.
For example, an employee at a US investment fund omitted the minus sign on a figure of $1.2 billion while transferring data between spreadsheets, thus “creating” a $2.3 billion gain instead of the actual $0.1 billion loss. The company announced to shareholders a distribution of $4.32 a share; a month later, when the error was detected, it had to announce that there would be no distribution (further information at Dr. Panko’s website; see “Other sites”).
The vast majority of such errors could have been avoided by following a small number of key quality principles in building models, as explained below. (References to spreadsheet features assume the use of Microsoft ExcelTM, but are equally applicable to other spreadsheet software.)