FAST 1.03-01 Do not split a model across multiple workbooks
FAST-1.03-01.1 except when more than one modeller must work concurrently
FAST-1.03-01.2 except when different files should be sent to different recipients
FAST-1.03-01.3 except when a single workbook would be too large and intimidating
Multiple, inter-linked workbooks are usually difficult to manage and generally a bad idea. However, the particular circumstances of a modelling project may sometimes dictate a so-called ‘split model’. Where timescales necessitate parallel model development by multiple modellers, where different recipients should receive different workbooks for considerations of confidentiality or specialty, e.g. a technical recipient not being confused by financing and tax logic, or where, despite the best efforts of heeding the principle of approximation, a single file would be intimidating and unwieldy simply for its size.
FAST 1.03-02 Avoid direct (external file) links
FAST-1.03-02.1 except when the logic flows back and forth between workbooks
It is generally simpler and easier to manage the manual interchange of data through dedicated import and export areas of respective inter-linked workbooks. However, direct links should be used when workbook A passes calculated values to workbook B, which in turn uses these values to calculate dependent values passed back to workbook A. However, such circumstances should call into question the decision to split the model.
FAST 1.03-03 Use import / export sheets for line items passed between workbooks
Data should be organised into a single ‘exported data’ sheet in the one file and an ‘imported data’ sheet in the second. Even if direct links are used, these are functionally similar to inputs, albeit potentially refreshed to different values via recalculations, and hence should be separated and organised with a similar approach.
FAST 1.03-04 External file links should be Named
If a model relies on links to external files, these should be named in the source file. In the absence of a call reference in an external file being named, Excel will be unable to keep track of any changes in the location of that cell in the external file.
In most instances, the FAST Modelling Standard advises against using Names, as stated in FAST 4.03-01: Do not use Excel Names, [page 42]. This is a notable exception to that rule.
