  Amongst the large number of write-and-throw-away spreadsheets developed for
one-time use there is a rather neglected proportion of spreadsheets that are
huge, periodically used, and submitted to regular update-cycles like any
conventionally evolving valuable legacy application software. However, due to
the very nature of spreadsheets, their evolution is particularly tricky and
therefore error-prone. In our strive to develop tools and methodologies to
improve spreadsheet quality, we analysed consolidation spreadsheets of an
internationally operating company for the errors they contain. The paper
presents the results of the field audit, involving 78 spreadsheets with 60,446
non-empty cells. As a by-product, the study performed was also to validate our
analysis tools in an industrial context. The evaluated auditing tool offers the
auditor a new view on the formula structure of the spreadsheet by grouping
similar formulas into equivalence classes. Our auditing approach defines three
similarity criteria between formulae, namely copy, logical and structural
equivalence. To improve the visualization of large spreadsheets, equivalences
and data dependencies are displayed in separated windows that are interlinked
with the spreadsheet. The auditing approach helps to find irregularities in the
geometrical pattern of similar formulas.
