iⁿ spreadsheet testing methodology — Instructions
Spreadsheets require clear instructions to ensure that they are used correctly. This includes instructions for the users of the spreadsheet, and also for the people who maintain and develop the spreadsheet.
Specific questions to consider when testing a spreadsheet's instructions:
- Are there clear instructions about how to use the spreadsheet? Clear instructions ensure that the user understands what the spreadsheet is for, how to use it, and how to interpret the results. Without clear instructions, the users may make incorrect assumptions and hence make incorrect decisions. The user instructions may be included within the spreadsheet and/or in separate documentation.
- Does the spreadsheet include instructions for developers? Instructions for developers (including those who maintain the spreadsheet) must specify particular special or unusual features to note, including special cases and potential traps for the unwary. Cell comments and VBA code comments should be used liberally. The developer instructions may be included within the spreadsheet and/or in separate documentation, though should be distinct from the user instructions.
- Are all the instructions up-to-date? As a spreadsheet evolves, it is easy for the instructions to become out of date. However, it is important that the instructions are updated as spreadsheet changes are made, to ensure that the instructions remain correct. Change control or version management may be appropriate in some cases.
- Are all data sources defined?
Spreadsheets require clear instructions to ensure that they are used correctly.When updating a spreadsheet, the user needs to know where to obtain data of the appropriate type and format. Both manual and automated data sources need to be fully specified.
- Does all data have units specified? For example, a cell may be called "Cost", with a value of "10" — but is that dollars per month, thousands of dollars per annum, a percentage of some other value, or what exactly? A related issue can occur when the basis of a piece of data is inadequately defined. For example, when a spreadsheet uses a time-value-of-money discount rate without stating whether the rate is before or after tax, or whether it is in real or nominal terms. In all cases the units should be defined adjacent to the data input.
Next we discuss the instruments that the spreadsheet uses.
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