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## Introduction

Excel is easy, right? Enter some figures, formulate some calculations and you’ve got immediate results. A spreadsheet is born. But then it grows, and grows, and… On average, spreadsheets continue to be developed for five years, and they are used by not just one or two, but thirteen different users. Not surprising then, that spreadsheets tend to get rather entangled. It’s been estimated that on average 7 billion dollars are lost each year due to errors in spreadsheets. Excel isn’t so easy after all!

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## Guideline 01: Document your Work

What separates the great spreadsheets from the other ones, is documentation! This will help future users of the spreadsheets to really understand your work. Even when the future user is you. Your spreadsheet should contain a ‘Documentation’ sheet as first, or last worksheet. Explain the future reader what the spreadsheet is for, where you obtained the data and where the results may be used.

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## Guideline 02: Never Use Fixed Numbers in Formulas

We have all seen “magic numbers” like this = SUM(A4:A12)*0.145. However, the use of magic numbers is a bad idea. A future user will not know where the number originates from. And a magic number does not change when needed, and thus might be overlooked when a change is made.

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## Guideline 03: Calculate Only Once

Sometimes, you need the exact same formula, based on the same numbers, in multiple places, like a calculation for inflation, on which you base several other formulas.

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## Guideline 04: 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. Remember that setup when you create a spreadsheet and follow this structure, both within a worksheet and between them.

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## Guideline 05: Delete What You Don't Need

Compare transferring a spreadsheet to renting your house via Airbnb. It remains more neat when you clean it first, right? Messy stimulates messy, neat stimulates neat. Remember an average spreadsheet will have 12 users after you! What would happen when the chain starts with a messy file?

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## Guideline 06: Do Not Merge Cells

It is so tempting! Merge cells in Excel so they form a header above two or more columns. Yes, it looks nice but resist the temptation because it can be dangerous.

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## Guideline 07: Use Consistent Labels

Numbers without meaning are worthless. Excel is a program for numbers, but it is only useful if it is clear to the reader or user what these numbers mean. So, make sure that you provide all data and formulas of textual information. Let’s call these information labels.

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## Guideline 08: The Dangers of Hiding Information

Do not hide anything except some sheets. Excel has too many nice options. In Guideline 6, we already advised not to use the option of merging cells, or only if really necessary. In this guideline we talk about the dangers of hiding information.

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## Guideline 09: Array Functions Are Past Times

Long ago, Excel was not as clever and advanced as today, but the so-called array formulas have been around for a long time. An array formula is briefly a formula that very cleverly combines multiple calculations as an array. For example, SUM (A1: A9 * B1: B9; 0) is the sum of (A1 * B1) + (A2 * B2) etcetera. However, people do not know them, and do not understand them well. Perhaps because of the very different nature of the formulas or the strange way of activating (CTRL-SHIFT ENTER instead of just ENTER).

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## Guideline 10: How To Manage Versions (And How Now To)

What version did I receive from you? Oh, I have modified the previous version. Has the board made a decision based on that version? Many nightmare has arisen because of confusion about the correct versions of Excel files. In the file itself, it is often difficult to see what the latest version is. And is the latest version the right version, or was it a “temporary break” to try something?

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## Guideline 11: Prefer INDEX and MATCH over VLOOKUP

There are several ways to search in Excel. The VLOOKUP and the combination of INDEX and MATCH are well known. To our great frustration, VLOOKUP is used a lot more often than the INDEX and MATCH, while the latter are really much better. INDEX MATCH are less error prone and a lot more efficient.

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