FAST 3.04-01 Do not write formulas with embedded constants
FAST-3.04-01.1 except when constants are universal
FAST-3.04-01.2 except when constants are deliberately embedded to avoid their manipulation
Embedding commercial information, for example an inflation rate, is never a good idea, as both clarity of model assumptions and active parameterization are lost. However, not all embedded inputs are created equal. Embedding a 24 (for 24 hours per day), 12 (12 months a year), 1000 (dollars in $ 000s) is permissible, even sensible.
As well, certain constants may be embedded as a clear design intent, rather than presented in explicit input cells. For instance, the model’s time frequency, e.g. 6 months per period, may not be able to be flexed, as the model may not have sufficient width to make this change, nor is the design ever expected to support such a change.
FAST 3.04-02 Include spaces between arguments in formulas
To the surprise of many modellers, Excel does allow the use of spaces in formulas. They make the formula clearer to read and only cause disruption in three places:
1. before the leading equal sign;
2. between a function name and the opening parenthesis; and
3. within compound logic operators such as <=, >=, and <>.
We note that the space itself can be used as the intersection operator. However, such usage should not be used, as it will almost always confuse the average model user.
FAST 3.04-03 Do not use parenthesis in formulas unnecessarily
FAST-3.04-03.1 except when they may improve formula clarity
Parentheses (‘brackets’ to the British) are used to separate the logic in formulas. Keep their use to a minimum, though in some circumstances Excel’s order of calculation is less clear than adding superfluous parentheses, for example ($a / 2) * b reads more easily for most people than the functionally identical $a / 2 * b
FAST 3.04-04 Use “-1 *” coefficient for all sign switches
Sign convention is a larger topic, but when sign switches are required, make the action as apparent as possible: -1 * a rather than simply -a. One cannot over-communicate the intention and visibility of sign switches.
FAST 3.04-05 Do not include current sheet references in formulas
Including a sheet label for cells referenced on the current sheet only serves to confuse formulas and the reviewer.
FAST 3.04-06 Do not use elements that appear to be non-structural in model logic
Using elements that would appear to be non-structural in model logic is risky as future users may not be aware of the importance of maintaining integrity. This would apply to using section headings, labels, or empty cells in model logic.
FAST 3.04-07 Do not over-anchor
Do not anchor beyond what is required to effect a consistent formula. Superfluous dollar signs not only clutter the formula from a reading perspective, but disrupt the ability to copy calculation blocks for re-use with different ingredient lines. See also FAST 2.02-02: Build calculation blocks so they can be replicated, [page 22]
