Separate Input & Output
When you read a book or a report or a website, we read from top to bottom and from left to right. Keep this in mind as you create your spreadsheet do your best to follow that structure both within each worksheet and between them. We advise you to distinguish separate input & output sheets as well as sheets for calculations, documentation and dashboards.
The Rule of Thumb
Have you ever gotten a visit from an accountant? Did they check that one important spreadsheet that contains your investment plans? Maybe they asked something like: What are the input values? What assumptions are these calculations are based on? Can you give an example of a calculation? Where are the conclusions?
This is just one of the moments in which you will benefit from a clear Excel model. The rule of thumb is: separate input & output. That is the first major step to really improving readability and maintainability. When we zoom into this further, we see the following as the categories we would generally group worksheets in:
Use 1 table per worksheet in input sheets. Make a differentiation between dynamic data that is gathered from outside systems, for example imported data from a CMR system, and data that can be found within the spreadsheet, such as calculations. Place references as close to the data as possible and make them easy to read, by using a color coding for example. This will make that accountant very happy and in general makes your spreadsheet easier to work with.
In the calculation worksheets you store information which does not change on a regular basis. These worksheets are often somber and purely functional. The purpose of these sheets is to use input data to draw conclusions. Remember that a spreadsheet stays in use for about 5 years, so even in these sheets things can change, just make sure the formulas are easy to read and adjust.
In the output sheets, we give an overview of the conclusions that can be drawn from the input data and calculations. Make sure you use tables, charts and nice layouts. Also think about making these worksheets printable by changing the margins, this is especially important for managers and decision makers.
There is one special category of worksheets meant for documentation. Use these sheets to explain the functionality of calculations and the source of input data. Other similar sheets to this are version control sheets and index sheets.
Some spreadsheet users love the use of so-called dashboards: worksheets that contain input, output and calculations together. While dashboards disrupt the flow of data, and the clear separation of input and output, they can be very insightful when modeling dynamic processes.
We do advise to minimize them, use maximum 1 or 2 (depending on the size of the spreadsheet) and make it clear this is a dashboard worksheet.
It is important to recognize different roles a worksheet can play, and make deliberate choices when creating a spreadsheet. We often encounter mixed sheets that are messy and confusing.
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