If the content is, say, B2 ( not =B2) then INDIRECT(“$A$1”) will return the value in cell B2. INDIRECT has an argument that is typically constructed out of text, eg, INDIRECT(“$A$1”), where, in this instance, the function will inspect the contents of cell A1.Some functions are obviously volatile, eg, NOW(), RAND(), TODAY(), and, perhaps slightly less obviously, CELL(“filename”), which keeps track of whether the file name has changed.
One test to check whether your workbook is volatile is to close a file after saving and see if Excel prompts you to save it a second time (this is an indicative test only). This occurs regardless of whether precedent cells/calculations have changed, or whether the formula also contains nonvolatile functions. More often than not, these instances are caused by volatile functions and/or volatile actions.Ī volatile function is one that causes recalculation of the formula in the cell where it resides every time Excel recalculates. However, modellers can do something about those functions and functionalities that Excel has potentially mistakenly considered dirty. Therefore, calculation time is a function of the number of dirty cells and the number of dependency trees, both affected and unaffected.Īs a modeller, you would find it difficult to change the number of dirty cells that genuinely need to be recalculated, short of writing more complex formulas in fewer cells (this is one reason “megaformulas” often calculate more quickly, although this increases the risk of modelling errors, etc, instead). When cells are changed, Microsoft recognises the cells that need recalculating as a consequence.
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Depending upon which version of Excel you are using, there may be a limit of how many dependency trees Excel can keep track of (it was 65,536 prior to Excel 2007, with the number debated by experts thereafter) before Excel has to resort to a full (ie, slower) calculation. These long chains are known as “dependency trees”. Rather than recalculate every cell every time any cell is changed, Excel determines which cells are affected by the latest change (known as “dependent cells”) and recalculates these dependents and then the dependents of these dependents and so on. The Excel calculation engine is almost as lazy - sorry, I mean “efficient” - as I am. Obviously, size does have some impact upon calculation speed, but perhaps not as much as you might think. What does that mean? I’ll tell you at the end, but first you need to understand what dirty and volatile mean in an Excel context. So what should modelers do? They should avoid making volatile moves that can dirty up their workbooks.
This action addresses only the symptom, not the cause. Although this action allows modellers to continue spreadsheet construction unencumbered, when was the last time you recall ever checking that a model you received was calculating automatically? Everyone just assumes that this is the case and makes managerial decisions accordingly.
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