Great Quotes about The Spreadsheet Problem™


Here are some useful quotes:
• Andrew’s first law of career spotting: Accountants are people who, when confronted with a problem, think “I know, I’ll use [A spreadsheet]”. Now they have two problems.
• [A spreadsheet] is a great prototyping tool, just don’t let it anywhere near your production systems.

While Panko provides a taxonomy of errors, it is focused on quantitative errors, where an answer is demonstrably wrong. In Teo and Tan, there is some discussion of qualitative errors, but the discussion is limited to "jamming" constants into a formula and duplicating values around the model.

Good Design.

Spreadsheet design is essentially the same as other software design. There are pretty well-established patterns, and a spreadsheet that follows well-established software design patterns is easier to work with than one that is haphazard.

In particular, a basic input-output pattern is critical to avoiding constants and duplication. The criteria for decomposing modules, based on allocation of responsibility, minimizing coupling and maximizing cohesion are all essential for avoiding duplication. In some cases, a Hidden Model™ page may be essential to cleanly separating inputs from reports.

Further, many of the basic usability design patterns can also be applied. A spreadsheet that looks like a document is useful for a final report. A spreadsheet that looks like an input panel is useful for the input side of the document. Mixing the inputs and the final report, while possible, isn't always the best strategy.

Spreadsheet as Document.

When we look at a spreadsheet as syntax of a document for capturing input from people, then we have to pay close attention to user-interface design patterns. In particular, most GUI's insist on labels preceding input fields. While this is largely taken for granted, anyone who has worked with a GUI builder knows that you have labels and input controls in an obvious juxtaposition. Usually labels are to the left of controls, but sometimes they are above.

In spreadsheets, we see the strange and poorly-thought-out documents. I think that a substantial number of "qualitative" errors can be stemmed with a few design guidelines. Here's an example that could be used to train users in a better way to use spreadsheets.
• Sheet 1 - Instructions
• Sheet 2 - Assumptions and Constants
• Sheet 3 to n - Inputs. These sheets look like forms with labels and fields. Some calculations are possible, as an aid to data entry, but the final answer isn't here.
• Sheet n+1 to the end - Reports. These sheets look like final documents. The calculations depend on Assumptions and Inputs.

We can, with a little XML and Python, validate the overall design pattern. We can locate constants in formulas, and improper references among spreadsheets.

Further, we can parse sheets 3 to n to extract the meaningful user inputs and put them into a more sophisticated, controlled, persistent and widely-shared repository.

The Hidden Model™.

In a few cases, a spreadsheet has outputs that come from the inputs via a tangle of intermediate results. Often that tangle is a business model that started out as one tidy report and then expanded out of control (usually via copy and paste) into a number of related reports.

Good software design (i.e., the MVC pattern) tells us that we have two views of the underlying model. We have the input parameters, we have a number of related reports. Between the two, we have the model itself, often a pretty simple set of calculations. The control is implicit in the way spreadsheets work; we just build the model and the views.

Posted: Wednesday - February 07, 2007 at 01:10 PM
       

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