Spreadsheets are notoriously error-prone.

Spreadsheets are often hard, if not impossible, to understand.

Mireault & Gresham (2015)

Every study, without exception, has found error rates much higher than organizations would wish to tolerate.

Panko (1999)

The results given by spreadsheets are often just wrong.

Sajaniemi (1998)

It is irrational to expect large error-free spreadsheets.

Panko (2013)

Research on spreadsheet errors is substantial, compelling, and unanimous.

Panko (2015)

Untested spreadsheets are riddled with errors.

Miller (2005)

Spreadsheets are easy to use and very hard to check.

Chen & Chan (2000)

Errors in spreadsheets... result in incorrect decisions being made and significant losses incurred.

Beaman, et al (2005)

Programmers exhibit unwarranted confidence in the correctness of their spreadsheets.

Krishna, et al (2001)

Spreadsheet errors... a great, often unrecognised, risk to corporate decision making & financial integrity.

Chadwick (2002)

...few incidents of spreadsheet errors are made public and these are usually not revealed by choice.

Kruck & Sheetz (2001)

Most large spreadsheets have dozens or even hundreds of errors.

Panko & Ordway (2005)

Spreadsheets have a notoriously high number of faults.

Rust, et al (2006)

Despite being staggeringly error prone, spreadsheets are a highly flexible programming environment.

Abreu, et al (2015)

A lot of decisions are being made on the basis of some bad numbers.

Ross (1996)

People tend to believe their spreadsheets are more accurate than they really are.

Caulkins, Morrison, & Weidemann (2006)

A significant proportion of spreadsheets have severe quality problems.

Ayalew (2007)

Spreadsheet errors have resulted in huge financial losses.

Abraham & Erwig (2007)

Spreadsheet errors are still the rule rather than the exception.

Nixon & O'Hara (2010)

Your spreadsheets may be disasters in the making.

Caulkins, Morrison, & Weidemann (2006)

The software that end users are creating... is riddled with errors.

Burnett & Myers (2014)

Never assume a spreadsheet is right, even your own.

Raffensperger (2001)

Spreadsheets... pose a greater threat to your business than almost anything you can imagine.

Howard (2005)

60% of large companies feel 'Spreadsheet Hell' describes their reliance on spreadsheets.

Murphy (2007)

It is now widely accepted that errors in spreadsheets are both common and potentially dangerous.

Nixon & O'Hara (2010)

The quality and reliability of spreadsheets is known to be poor.

Bishop & McDaid (2007)

Even obvious, elementary errors in very simple, clearly documented spreadsheets are... difficult to find.

Galletta, et al (1993)

Spreadsheet shortcomings can significantly hamper an organization's business operation.

Reschenhofer & Matthes (2015)

Spreadsheets are extraordinarily and unacceptably prone to error.

Dunn (2010)

Every study that has looked for errors has found them... in considerable abundance.

Panko & Halverson (1996)

Studies have shown that there is a high incidence of errors in spreadsheets.

Csernoch & Biro (2013)

Errors in spreadsheets are as ubiquitous as spreadsheets themselves.

Colbenz (2005)

The untested spreadsheet is as dangerous and untrustworthy as an untested program.

Price (2006)

Spreadsheet errors are pervasive, stubborn, ubiquitous and complex.

Irons (2003)

Spreadsheets can be viewed as a highly flexible programming environment for end users.

Abreu, et al (2015)

Spreadsheets are commonly used and commonly flawed.

Caulkins, Morrison, & Weidemann (2008)

Spreadsheets contain errors at an alarmingly high rate.

Abraham, et al (2005)

Overconfidence is one of the most substantial causes of spreadsheet errors.

Sakal, et al (2015)

The issue is not whether there is an error but how many errors there are and how serious they are.

Panko (2007)

Spreadsheets are dangerous to their authors and others.

Durusau & Hunting (2015)

Spreadsheet development must embrace extensive testing in order to be taken seriously as a profession.

Bock (2016)

Despite overwhelming and unanimous evidence... companies have continued to ignore spreadsheet error risks.

Panko (2014)

Spreadsheets are the most popular live programming environments, but they are also notoriously fault-prone.

Hermans & van der Storm (2015)

1% of all formulas in operational spreadsheets are in error.

Powell, Baker, & Lawson (2009)

Spreadsheets are alarmingly error-prone to write.

Paine (2001)

Developing an error-free spreadsheet has been a problem since the beginning of end-user computing.

Mireault (2015)

Most executives do not really check or verify the accuracy or validity of [their] spreadsheets...

Teo & Tan (1999)

94% of the 88 spreadsheets audited in 7 studies have contained errors.

Panko (2008)

Spreadsheets are more fault-prone than other software.

Kulesz & Ostberg (2013)

Blog

This is an occasional blog about spreadsheet best practice, spreadsheet errors, and testing of spreadsheets.

Spreadsheet good practice guidelines

Check out our "Spreadsheet good practice guidelines" booklet on Amazon.

Good spreadsheets are essential for informing decision making. But most spreadsheets are not good. The objective of this booklet is to help you make better spreadsheets, so you can make better decisions.

The spreadsheet good practice guidelines consist of five one-page modules, each of which summarises a key aspect of making better spreadsheets:

Design principles. Deliberately design every spreadsheet, applying the spreadsheet design principles and their supporting questions.
Spreadsheet Development Life Cycle. Every spreadsheet goes through distinct phases during its life, from Planning to Archive. Manage the life cycle and allocate appropriate time to each phase.
Top 10 good practices. Applying these good practices will help you make better spreadsheets.
Top 10 most common errors. Avoid these errors to improve the quality of your spreadsheets.
Test, test, test. All spreadsheets must be tested. Learn how to test a spreadsheet, applying our detailed testing procedure.

These guidelines are distilled from many years of practical experience in designing, building, and testing spreadsheets, supported by an extensive review of the relevant academic literature.

The booklet contains about 3,000 words on 10 pages. Buy now

Making better spreadsheets

Spreadsheets are riddled with errors. To reduce the risks our practices must improve.

The objectives of this presentation are to:

Highlight the risks of heedless spreadsheet development.
Outline ways to reduce those risks.

We provide guidance based on:

Hard-fought practical experience.
Academic literature on spreadsheet risks and best practice.

Improving spreadsheet run time

An important aspect of a spreadsheet's fitness-for-purpose is ensuring that its run time is acceptable to the users. Creating an inefficient spreadsheet is easy, while creating an efficient spreadsheet requires a bit more thought and the application of some techniques to reduce run time.

This article presents an example of using refactoring techniques to vastly improve the run time of a spreadsheet. We focus on an especially slow feature of Excel: the interaction between VBA and the worksheets.

Calculation cascade: A common cause of catastrophe

Trivial sales spreadsheet - with an error

People are remarkably accurate when doing a wide range of activities, including working with spreadsheets. Even so, the bottom line result of every spreadsheet is almost certainly wrong.

For example, if you have a spreadsheet consisting of 100 used cells and a moderate 3% probability that each cell contains an error, then the probability that the spreadsheet's results are wrong is 95%.

This article explores how and why the cascading structure of spreadsheets leads to a high error rate, and then we consider what we can do about it.

FizzBuzz in Excel: Why spreadsheet documentation is important

FizzBuzz is a simple word game that is sometimes used as a test for programmers. Here we use FizzBuzz to illustrate the importance of documenting your spreadsheet.

Summary of key points:

There are often many ways to write a spreadsheet formula.
Documentation is a very important, though often neglected, part of every spreadsheet.
Including documentation is important because it makes a spreadsheet easier to understand and use.
Types of documentation include cell comments, an 'About' worksheet, and text boxes.
Cell comments can be especially useful for describing how a formula works and pointing out subtleties that might otherwise not be obvious to the user.
Ensure that the documentation is updated as the spreadsheet changes.

Pitfalls of Excel's NPV function

Incorrect use of Excel's NPV function is a common source of spreadsheet errors. This article discusses several pitfalls and suggests ways to avoid them.

Summary of key points:

Excel's NPV function is a common source of spreadsheet errors.
Even simple spreadsheets often contain material errors.
When using the NPV function, errors that commonly occur include: referencing the wrong cells, constant jamming, inadequate documentation, inconsistent assumptions about cash flows being real/nominal and before/after tax, misinterpretation of an ambiguous model specification, and incorrect cash flow timing.
Finding and correcting errors requires inspecting every cell for errors to ensure that assumptions are consistent, functions are used correctly, good practice is followed, and the spreadsheet appropriately represents the situation being modelled.

Your spreadsheets are wrong

In this article we describe how spreadsheets are riddled with errors, drawing on our own experience and on the research presented in the academic literature on spreadsheet errors and testing.

Around 95% of all spreadsheets contain errors. So, unless you've been extremely diligent, or perhaps just very lucky, then it is almost certain that your spreadsheets are wrong.

The spreadsheets that you rely on — to inform decision making, do reporting, and generally conduct analysis of just about everything — are probably giving you incorrect results.

iⁿ spreadsheet testing methodology: Conclusion

In this series of articles we introduce the iⁿ methodology for testing spreadsheets.

The iⁿ spreadsheet testing methodology is a set of guidelines for validating that a spreadsheet does what it is intended to do and ensuring that it will continue to do so.

The iⁿ methodology consists of five components:

Intent.
Instructions.
Instruments.
Implementation.
Immunity.

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