Daniel Lemire's blog

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You shouldn’t use a spreadsheet for important work (I mean it)

61 thoughts on “You shouldn’t use a spreadsheet for important work (I mean it)”

  1. Sam Bowne says:

    I worked as a DBA at a financial company for years and I learned the exact opposite: Always use spreadsheets for financial work, unless you have large tables or several tables to connect, then use databases. However, you must use them correctly: each column contains one more step of the calculation, so it can be audited by hand for chosen records with a calculator.

    Spreadsheets are designed for financial documents and they work very well for that.

  2. @Sam

    What Piketty was doing was merging lots of different data sources, interpolating missing data, and so on. Very complex number crunching that spreadsheets were not designed for.

  3. John DOugan says:

    If Barak Obama can get a Nobel prize without actually having done anything peaceful up to that time, I suspect that the “Sveriges Riksbank Prize in Economic Sciences in Memory of Alfred Nobel” committee might not care about the actual correctness.

    1. Kit says:

      Sweden doesn’t issue the peace prize. Stay ignorant.

  4. Ivan says:

    You are very very naive if you think this mistakes are not on purpose. Socialism sells in leftist media(also known as all the media minus neocon FOX) so he did what he needed to do to become next hero of the MSM.
    Ofc it is possible that he is just an useful idiot also.

  5. I completely agree with you, of course.

    One small point in defense of Piketty: I am actually reading his book, and although I am not an economist, I find it very readable and full of interesting points of view. One thing that Piketty says at the very beginning is that what is important is the trend, and not the actual precise values, because in effect there is no way to get the actual precise values. So, it is all a matter of how large is the statistical error. The first chapter contains a long critical analysis of what does it mean to collect these data, and a computer scientist may find appalling the amount of approximations that it is necessary to do to get anywhere in this kind of studies.

    I do not know what he did wrong, and surely he should have used more professional statistical tools for analysing the data. At the same time, it is vane to think that it would be possible to find the “truth” in studies like these. Economic is still more closely related to politic and sociology than it is to mathematics.

  6. Frédéric Godin says:

    If spreadsheets are used, visual basic macros on Excel can be used to automate the process. This gives the best of both worlds: viewing results is easy due to the spreadsheet display, but the code is still centralized and can be verified more easily.

    1. Paul Podbielski says:

      I second this. User functions are the only thing that makes Excel tolerable.

  7. mtraven says:

    I’ve worked on efforts to make spreadsheet-like systems that were smarter (worked at a higher level so less prone to stupid manual errors). I used the Reinhart-Rogoff example in a grant application. So I’m well aware of the problem, but I think you aren’t right to yell at Piketty for not writing his own software, the blame is on software makers for making crap tools that lead to mistakes. Why should we expect an economist to also be a professional grade programmer?

  8. First, I am inclined to cut Piketty a lot more slack. First, he claims to have put all his data in public, and welcomed review.

    Just by itself, that is a very positive sign.

    (Bought his book, but not yet read. He got a highly positive review from a mind I trust.)

    Second, he is trying the make sense of a rather motley collection of data. No alternative, in this case.

    Last, there are a lot of folk using spreadsheets, with all their weakness, for lack of a better alternative.

    (The story of Lotus Improv is relevant. It should exist, but does not.)

  9. Henk Poley says:

    I wonder how much can be fixed by changing the UI. Some companies such as infotron.nl and useslate.com are trying to visualize the information flow and bugs in spreadsheets.

  10. @Lipari

    One thing that Piketty says at the very beginning is that what is important is the trend, and not the actual precise values, because in effect there is no way to get the actual precise values. So, it is all a matter of how large is the statistical error. (…) At the same time, it is vane to think that it would be possible to find the “truth” in studies like these. Economic is still more closely related to politic and sociology than it is to mathematics.

    It is not uncommon for scientists to have to deal with high margins of errors. They do end up, sometimes, concluding that the results are inconclusive. This is not what Piketty did… he found that the data was conclusive so he felt that the margin of error was sufficiently small.

  11. Len Pannett says:

    It may not be his conclusions that are incorrect, it is Piketty’s approach to having errors pointed out to him that is. When Andrew Wiles was shown that there was an error/omission in his proof of Fermat’s Last Theorem, did he tell his detractors, “You show me otherwise”? No, he cracked on and corrected that proof.

    Piketty would have shown more judgement and rigour by reviewing the data and its conclusions in light of the highlights from the FT (and others), looking to see how it materially affected the conclusions and re-publishing if necessary and taking the concept of peer-review with the humility that it deserves. Had he done that, there would be no doubts of potential Nobel-worthiness.

  12. @Pannett

    looking to see how it materially affected the conclusions and re-publishing if necessary and taking the concept of peer-review with the humility that it deserves

    It is wise advice but… when you publish a 600-page book out of original research… you are in a tough spot if mistakes are found. He simply cannot revise the book in a timely manner. He is stuck with it as it stands now for many years.

    Of course, he chose to publish a long book based on original research…

  13. JJG says:

    In particular, on should not use Excel for any computations, since it cannot correctly calculate a standard deviation:


  14. SomeDude says:

    Had a customer last week doing biotech research with excel, processing data logs in the gigabytes. Because of intitute policy. She knows R, python, matlab and some crude C, but because “the environment is very international”, they settled for Excel.

  15. Martin Haeberli says:

    Nice article, but:

    In addition to numbers, natural language (here, English) has its challenges, too. Example: “Two famous economists had concluded that high debt lead to slow economic growth based on an extensive statistical analysis of historical data.” reads better as “Two famous economists had concluded that high debt led to slow economic growth based on an extensive statistical analysis of historical data.”
    TL;DR “lead” should be “led”

  16. Foo says:

    Devil’s advocate:

    When I’ve had to deal with “sane software” rather than spreadsheets, in 100% of cases when I’ve dug up the research paper with the original algorithm, the software had at least one elementary transcription bug. As you rightly point out, it’s not about being bug-free (which is virtually impossible), but whether spreadsheets as a tool are better or worse at this than other kinds of languages.

    Spreadsheets make testing hard, make code review difficult, and encourage copy/paste coding. Apparently normal programming languages are just the same, because I can go on Github and find that *most* source code, across all languages, is not tested, has not been reviewed, and is full of copy/paste sections, too.

    What is your threshold? For example, how much copy/paste does a language need to exhibit, in practice, before it’s bad? If we find that (hypothetically) Python users copy/paste as much as spreadsheet users, should we stop trusting results from Python programs? (Maybe the problem is not the tool but the economists writing software, and so telling them to stop using spreadsheets means they’ll just carry over their bad habits to other languages.)

    Just as you put the burden on spreadsheet users to demonstrate that their processes are as good as other programming languages, why isn’t the burden also on you to show that it’s worse? *Is* it worse? Is it bigger than the difference in robustness between, say, C++ and Lisp?

    I share a gut feeling that spreadsheets are terrible, but I don’t see this as an indictment of them. You’ve shown me nothing here I don’t also see in every other programming language ever.

  17. Bill Conerly says:

    I’ve made errors using econometric software and writing my own programs. It’s not spreadsheets per se that lead to errors. And programming is harder for the casual user (like an economist) than it was in the days of BASIC. Those of us who use spreadsheets where they are appropriate get comfortable with the tool and then push it into areas it probably shouldn’t be pushed–but that’s easier than struggling with a rarely-used programming language.

    Back in the 1980s I read something about Lotus 1-2-3: “Every complex spreadsheet has an error.”

    With that in mind, a spreadsheet creator can add lots of internal validity checks. I also pull out a calculator to hand-check some random results. I try not to copy data, but instead to link to source data. These techniques are not foolproof, but they help a lot.

  18. Paul Dulaney says:

    I came to this page thinking it was a joke, but you’ve made an excellent case! And if I compare an Excel spreadsheet with a Matlab source file, one big difference jumps out: the Matlab file has a clear linear progression from beginning to end. You simply cannot trust a human being to effectively review a two-dimensional array of equations with all kinds of random linkages.

  19. @Martin


  20. @Foo

    Many Python or R programs are not tested or reviewed and rely extensively on copied code. And that is perfectly fine. Nobody is going to care if you get it wrong.

    However, if you write a 600-page book and believe you are going to be up for the Nobel prize… then the bar is higher… Your code should be tested, documented and reviewed.

    Excel does not allow any of these things.

  21. D says:

    …shocking how much investment banks abuse Excel with many desks running intra-day risk off of spreadsheets, and PC cludging together rec’s and analysis… its a much abused tool.

  22. Ashley says:

    A few points from a financial market economist with some experience in R:

    You use the tool that everybody else uses, otherwise ideas cannot be communicated effectively. If had published his work in R, the vast majority of economists, including the reviewers at the FT would not have been able to review his work.

    Excel is very useful for combining different data sets of different frequency and often irregular frequency. This would be a nightmare in R.

    Generally basic data manipulation, which is actually the bulk of most interesting work in economics, can be done in excel. Other tools, such as R or Eviews, are used when more advanced statistical techniques are needed.

  23. J. J. Ramsey says:


    Copying and pasting maybe done with other programming languages, but spreadsheets practically *demand* copying and pasting just to get formulas into the cells. Worse, as there are more and more cells, more and more copying and pasting has to be done, far more than would be needed with an actual programming language. Furthermore, when writing actual program code, one can see where one is starting to repeat oneself via copy-and-paste. If one catches that repetition early, then it’s not too difficult to refactor by replacing the copy-paste code with a suitable function that contains the code that had originally been copied and pasted. You can’t do that in a spreadsheet.

  24. sneedy says:

    ‘In his spreadsheets … there are transcription errors from the original sources & incorrect formulas.’

    and ‘code’ magically prevents this?

    as for testing, there’s nothing in Excel that prevents you from testing a formula. in either case (spreadsheet vs code) you have to understand your formulas and know what results to expect.

    for anyone but an experienced and highly skilled coder, writing code it as prone to error as using a spreadsheet, maybe moreso. for manipulating tables of data, a spreadsheet is easier for the average person than writing code by several levels of magnitude. why would i write a GUI to manipulate tabular data when someone’s already done it? why would i learn how to code complex formulas, rounding rules, display rules for various number formats, etc etc when someone’s already done it – and it’s been tested over a couple of decades by millions of users?

  25. Paul Hammant says:

    Granted the spreadsheet in question is huge, but I wonder if http://paulhammant.com/2012/08/10/its-time-for-a-new-take-on-spreadsheets/ paints a better foundation for such work.

  26. Tom says:

    I doubt it would have made much of a difference in this case. Even with other kinds of software, users have a distressing habit of introducing bugs that lead to results favoring their hypotheses and removing bugs that lead to results contradicting their hypotheses.

    Although it’s nice that people have tracked down specific errors in Piketty’s numerical analysis, I don’t think those are even the worst part; even his basic assumptions don’t make much sense.

  27. phayes says:

    “Like Reinhart and Rogoff, Piketty does not deny that there are problems, but (like Reinhart and Rogoff) he claims that his mistakes are probably inconsequential…”

    Well he’s probably right and
    insofar as Reinhart and Rogoff were referring only to their spreadsheet errors (and not e.g. their glaring causality attribution error), they were right too. But of course such errors easily could be consequential and I agree that “spreadsheet science” is a problem. There’s another good article on the subject here: http://www.burns-stat.com/documents/tutorials/spreadsheet-addiction/

  28. Rich Plevin says:

    Of course, it’s possible to write crappy code in any language. The point is rather that some environments make it easier than others to write robust, maintainable code. Excel isn’t one of them.

    Another issue I’ve encountered with Excel (and other environments that generally combine data, program, and GUI in one monolithic file) is that it’s very difficult for multiple developers to work simultaneously, to build reusable components, and to use source code control system. Anyone who has worked professionally in software development would recognize these as serious limitations for all but trivial projects.

    Of course, one can code in VBA within Excel, but VBA is not a very efficient for software development. This probably isn’t apparent to folks who have worked only in VBA. There are half a dozen languages I’d rather use…

  29. Mark Pawelek says:

    This is a peer review problem not a spreadsheet problem. Economists don’t have the same tradition of peer review as scientists.

  30. shuggie says:

    I used to work for an electricity supplier maintaining software for a quotations system for commercial customers. In essence what it did was provide bespoke prices based on estimated uses drawn from half-hourly meter readings. So whereas a domestic customer may get their meter read once every few months if they’re lucky, these companies had meters recording readings 48 times a day.

    So what the system did was take the last set of known readings, extrapolate them over the next contract period (adjusting so the days of the week and public holidays matched), factor in some of the add-on charges and try to predict costs based on that. In mathematical terms this meant a lot of manipulations of matrices with 48×365 or 48×730 values.

    This was conventional software but how to test? Well there was a guy in the contracts department with a spreadsheet. Whenever our figures didn’t match we had to figure out if it was a bug in our code or his spreadsheet. We found plenty examples of both.

    And the irony of it all was that most times these figures were the starting point for negotiations. So the importance of the “accuracy” of the results was moot. It was primarily a way of giving a customer an impression that we were really rigorous and detailed with figures and therefore must be giving them the best possible deal. Whereas in fact the way to get the best deal is same as always – negotiate as hard a possible and play suppliers off each other to find the lowest quote.

  31. Vad says:

    I work in research. With any kind of modeling or statistics you need a statistician. There is a proper process for predictive modeling and trends, it’s not something you just whip up yourself as an economist regardless what software you use.

  32. SR says:

    a big inheritance is less likely to land you in the list of the richest people today than in 1970…

    Fascinating, has the population changed since 1970 where are your numbers and code? Seems like you are responding to a fairly serious work with a lot of fluff because you disagree with it.

  33. Indig-Gestion says:


    You call for better tools and for [better] techniques. But are they flawed in equal proportion? You’ve left that point ambiguous, yet I think it’s an important distinction. Curious to know your view.

    Spreadsheets represent a resource tradeoff. To borrow from @D’s comment above, as I write this I am “running intra-day risk off of spreadsheets”. This is because I cannot afford a better solution if I want to keep my costs low enough to deliver competitive investment returns to my clients. The big banks don’t have that excuse at the institutional level, but as the “London Whale” episode illustrates, even JPM was making economies on risk management. Speaking from experience that is entirely commonplace.

    Academic researchers probably have relatively less demanding deadlines but we all have families and time is precious. Tradeoffs will be made. So if this is an optimization issue, are we first to solve for the tool or for the technique(s)?

  34. Björn says:

    Use spreadsheets to get the all calculations correct.

    Then move it to a real programming language to make a program that you can put in a automated production line, that is automatically tested, that integrates well, is fast and nobody will mess it up by accidently changing some random cell.

    That why accounting isn’t done in Excel for example.

  35. Martin says:

    @John DOugan
    The Nobel Peace Prize is decided on by Norwegian Politicians. The other Nobel prizes are decided by scientists. You can’t really compare the two.

  36. Meburke says:

    Theoretically, once a programming problem is solved it is never necessary to “re-invent the wheel” by inventing your own code to solve the problem. For years, the ACM has published all the fundamental algorithms in one form or another. Modern programmers are using code that programmers, such as myself, invented back in the 60’s and earlier. Cut and paste programming is not always such a bad thing.

    But errors creep in at other places: The Floating point processor can make errors; there can be memory errors, compile errors, and language deficiencies. Since so many systems are interdependent on multiple sources of software, the accumulation of errors can be disastrous.

    Excel made extensive changes to it’s probability and statistics fopr 2010, but there is still a maze of complex systems.

    All that said, the conclusions prought about by economists are a matter of logic, not statistics. Precision at lower levels should not affect the logical conclusions unless there is a major distortion in the analysis.

  37. Brian Knoblauch says:

    Having been an IT guy for a long time now, the one biggest change I would like to make is to make spreadsheets illegal except for specific circumstances. What I’ve found over the years is that not only are they used for the wrong problems, they’re almost always used incorrectly as well. Errors abound. Software in general is pretty buggy, but spreadsheets thrown together by end users are absolutely horrible. Stop the madness, uninstall spreadsheet software!

  38. John McKown says:

    Very interesting article. I sent the URL to one of our actuarials. I really doubt he’ll agree. One thing especially mentioned was Excel. As an alternative, I would suggest that perhaps it would be easier to audit a spreadsheet if people would use Libre/Open Office. The reason is that the file format is documented. Basically, the data are contained in XML files in a zip archive. This means that it is possible to transform this data into a different format (using XML tools), which would be easier to read and audit.

  39. Adrian Walker says:

    There’s a small study showing how Reinhart-Rogoff and others might have caught their errors by writing instead in Executable English.

    The study is here:

    and here is the “code”:

  40. Spreadsheets certainly have a bad reputation and there are no shortage of spreadsheet horror stories (we did our own rather tongue in cheek round up in this ebook – http://info.f1f9.com/dirty-dozen-12-financial-modelling-horror-stories-ebook)

    Spreadsheets are software, and all software development requires discipline. The FAST Standard Organisation is making good progress introducing a disciplined structured approach to spreadsheet model development. Check out http://www.fast-standard.org.

  41. Mark Pawelek says:

    The only way you can guard against mis-transcribed data is to have two people do the transcription independently to create files with the same format. Then take the difference, or otherwise detect differences. This is what they did for data input long ago during the “data processing” era of the 70’s and 60’s.

  42. Tafari says:

    Blaming tools is a futile endeavor. The issue is rarely the result of the tool utilized. Reaching back to the analogy of the surgeon, a scalpel is an inactive piece of metal. However, no one ever blames the scalpel when a surgery goes awry. The success of an endeavor always comes down to the skill and due diligence of the individual approaching the effort. As a former financial analyst and current software engineer I can emphatically say that tools are helpful and may contribute or detract from intended outcomes, but the human being behind the tool is always the key factor in whether a quality result is achieved. (Typed and spell-checked in Microsoft Word).

  43. @Tafari – “blaming tools is a future endeavor” – totally agree with your comment.

  44. By the way, Piketty has responded officially:


    and claims that FT experts made an arbitrary choice in their counter-analysis. Now it becomes too technical for non-economists like me, but I want to remark that the matter is much more complicated than it seems.

  45. @Lipari

    I allude to his response in my post. I find it lacking. He puts the burden of proof on his critics. This is not how science works. He has the burden of proof.

  46. The one you allude to is the quick response to the journalist after the FT published its concerns, and certainly it is lacking. This response I linked is a 10-page document with links to data:


  47. Adrian Walker says:

    Hi Guiseppe,

    You wrote: “Now it becomes too technical for non-economists like me, but I want to remark that the matter is much more complicated than it seems.”

    Well, yes. But if written in Executable English, people with varying amounts of expertise can understand and comment. Plus, they can get step-by-step English explanations of the execution results.

    The “code” for an RR-like example is here: http://www.reengineeringllc.com/demo_agents/GrowthAndDebt1.agent

  48. A.Lahiri says:

    Thank you for a very interesting article, Daniel, and the comments were quite informative too.

    Tools can influence outcomes.

    IMHO, whatever mistakes were made in the spreadsheet would have been more visible and hence likely to get fixed, if something like R was used, although I see Ashley has said R can’t easily handle sparse/irregular data.

    Perhaps Python with Pandas or some such?

    Or a Timeseries DB?

    Adrian’s Executable English comment is intriguing – will definitely take a look.

    Also, not entirely OT, you may have heard of Tufte’s comment on NASA’s use of Powerpoint:


  49. Matt Pettis says:

    I think the bigger idea is that researchers of all stripes adopt the ideas of ‘reproducible research.’ A start for the ideas here:


    The big idea to me is that all manipulations with data leave an audit trail, and you need a system (software) that supports this. And this is the main difference between R/SAS/Python-Pandas and Excel: all of the non-Excel examples really allow you to record *in code* what is being done with the data. Well, as long as you force yourself to never edit the underlying data, but only remove/edit data via code statements. This allows many errors to be surfaced purely because they are explicitly laid out in code. Compare that to moving data around in Excel by dragging and dropping. There is no way to record that motion in Excel, but there is in the other languages.

    There is much more to it than that, but to me, that is the first big advantage…

  50. Matt Pettis says:

    Also, hand in glove with Excel problems, I think of this animated true story… https://www.youtube.com/watch?v=N2zK3sAtr-4

  51. Matt Pettis says:

    … I should also add that Daniel did address this here, but I just wanted to put the the google-able topic name that address is this with this post…

  52. Donakd Parish says:

    ”Modern’ Excel using the Power BI add ins such as Power Pivot using the DAX formula language with pivot tables can help. It gives you what Rob Collie calls ‘reusable formula’. Trust me, I’m a programmer 🙂

  53. Ken Taylor says:

    Code and spreadsheets can contain errors and be written to be indecipherable or more easily reviewable. My view, based on experience, is that it is just as likely with either method for novel analysis. Both methods are also amenable to manipulation to achieve desired results.

    You say “it appears that Piketty’s code contains mistakes, fudging and other problems” and link to discussions of some. I followed the link to Magnus who provides a detailed discussion of the problems he observes in the spreadsheet. I see this as evidence against your claim that spreadsheets are not reviewable.

    My view may be wrong, but I see only assertions in this article to contradict it.

  54. E.L. Beck says:

    But even STATA cannot cope with assumptions initially poured into the formulas, or the blindness we all labor under with these assumptions.

  55. @Ken

    You can review Excel spreadsheets, my point is that “spreadsheets make code review difficult.”

    The only reason Piketty’s spreadsheets are being reviewed (by a few key individuals) is because he is holding in the best-seller list.

    But look at how difficult these reviews are. People have to attach screenshots to make their points… and, even then, it isn’t very clear…

    You wouldn’t want the people who write software for the space shuttle to work this way.

  56. Daniel said: “You can review Excel spreadsheets, my point is that “spreadsheets make code review difficult.”

    Adrian says: +1

    To Ken: a more radical fix is to write the data extraction in executable English [1]. You can then get a human-readable audit trail.

    Here’s an example: http://www.reengineeringllc.com/demo_agents/GrowthAndDebt1.agent

  57. Matt wrote: This is the main difference between R/SAS/Python-Pandas and Excel: all of the non-Excel examples really allow you to record *in code* what is being done with the data.

    Adrian agrees and says: Where possible, writing in *Executable English* goes further. In particular, it supports human readable explanations/audit-trails of what data were used and what the app has done with the data.

    Example: http://www.reengineeringllc.com/demo_agents/EnergyIndependence1.agent

  58. Jeff Weir says:

    @Daniel: Spreadsheets don’t make code review difficult. *Bad* spreadsheets make code review difficult. As does bad VBA, SAS, Python, R, or whatever language someone abuses.

    You keep saying that you wouldn’t want the people who write software for the space shuttle or the banking industry to be writing codes in spreadsheets. Funny thing is, they do. And sometimes they make mistakes. Mostly human error. And I’m pretty sure they make mistakes no matter *what* technology they employ.

    1. There are standard tools and techniques to test and review software written in Python, R, Java… How do you even test and review Excel spreadsheets? If someone modifies a function in Excel, how does it get reviewed and approved? What is the testing protocol?

      Of course, maybe NASA has morons running their computing systems. I would not be surprised. But that’s hardly an excuse to do the same.

      And sometimes they make mistakes. Mostly human error. And I’m pretty sure they make mistakes no matter *what* technology they employ.

      Almost all car accidents are due to human error. There are car accidents and fatalities no matter which car we drive. So let us go back to cars from the 1950s without safety features.


      We can build much safer roads and cars than what we had in the 1950s.

      The same holds true for software. We can build much better software by following better protocols and using state-of-the-art techniques.