Errors and Problems

Dealing with Circular References

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Formulas discussed: Algebra, VBA (Copy, Paste, Select, Range, If..Then..End If, For..Next
The workbook used throughout this post is available here.

The problem

When building a model, it is not unusual to, either accidentally or deliberately, build a circular reference. A circular reference is where the result of a formula depends on itself. This could be as simple as B5 = B4+B5, or a complicated loop where cell A references B, which references C, which references D, which references B and E, while E references A, as in the diagram below:

Brendan WalpoleDealing with Circular References
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Offset and Choose

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Formulas discussed: Offset, Choose, Lookup, Index
The workbook used throughout this article is available here.

Both Offset and Choose are commonly used in scenario/sensitivity selections. Unfortunately, both functions have major weaknesses, even when used in something as simple as scenario selection.

Brendan WalpoleOffset and Choose
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Even Microsoft makes Excel mistakes

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You can see the original article by David Yanofsky on Quartz here.
Formulas discussed: Sum

This photo is from an ad for the new Microsoft Surface tablet, meant to show some of the things you can do on the new tablet (photo by David Yanofsky)
microsoft_math_detail

What most will notice is that the Total in cell B9 is showing $9,000, but adding up the individual holiday items comes to $9,500. Strangely enough, the pie chart (despite being one of those horrendous exploding ones) looks to be correct. At least it wasn’t a 3D exploding pie.

So what happened. Some peoples guesses on Twitter were that B3 was being edited, but it appears in the chart already.

Brendan WalpoleEven Microsoft makes Excel mistakes
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Net Present Value and Internal Rate of Return

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Formulas discussed: NPV, IRR, XNPV, XIRR
The workbook used throughout this article is available here.

Net Present Value (NPV) and Internal Rate of Return (IRR) are thought to be the two of the most common tools for evaluating the financial viability of projects. What is also apparent is that there is significant debate as to the appropriateness of these measures, which is something we’ll leave to those other forums (such as here and here).
What we will cover in this article is their incorrect use in Excel, due largely to people misusing the inbuilt formulas or not understanding their mathematical construction. This is often apparent in financial models, where the a single outcome is sought, and the formula syntax is simple to use, but the modeller may not completely understand what is going on under the hood of these formulas and produces an answer that is not what they intended (whether they detect the error or not).

Brendan WalpoleNet Present Value and Internal Rate of Return
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Index and match versus lookups – when to use them

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Formulas discussed: Lookup, Vlookup, Hlookup, Index, Match.
Just want the table to show you which formula to pick – click here. The workbook used throughout this post is available here.

This article and the attached spreadsheet will discuss the various methods of searching for a data point in an Excel table and return the value in a corresponding row or column, without having to manually look through the table yourself. Common examples include searching for a sales result for a particular period, or matching a data field to a person’s name.

screen shot - sorted data table

Brendan WalpoleIndex and match versus lookups – when to use them
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