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Once I was in a meeting where someone was presenting from a spreadsheet. As the meeting was insanely boring I was entertaining myself double checking the math i

Once I was in a meeting where someone was presenting from a spreadsheet. As the meeting was insanely boring I was entertaining myself double checking the math in the spreadsheet and spotted a formula error thanks to mental math. I don't recall that it changed anything, but the amount of smugness I felt when I pointed out the error and the presenter confirmed it was so great that I have a smug smile on my face even now, five or six years later, just thinking about it.

conductr 5y ago
It happens. I work in corporate finance and am presenting boring spreadsheet stuff daily. I/My team makes mistakes too and I don’t always catch them before the presentation. Sometimes there’s literally no time it’s like dev on prod without tests. Anyways, what you did is very common. Many people are actively looking for mistakes in the math instead of focusing on the content of the presentation. And will derail my entire meeting when they find one. Just word of warning , don’t be that guy. Usually it’s some unimportant metric (or someone would have noticed) and directional correctness facilitates the same conversation. For example, oh you’re right overtime is running at 10% instead of 8% is usually not a material fact when your department is 30% over budget and some productivity metric is where the businesses focus needs to be (eg. widgets/hour). Instead, bring it to my attention after the fact. Or shoot me a email while in the meeting. Only verbalize it if it’s crucial to the conversation or impacts the decision being made. It’s a bit like expecting bug free software, it’s just impossible. Not saying you handled it wrong perse, most people do this. Just suggesting how I have been on the other side of this and prefer for others to act. I find this more professional as I wouldn’t turn up to your product launch party and just tell everyone in the room about the GitHub issues log. Or, as another example, if I were your manager, I wouldn’t call you out publicly but assume you prefer a private feedback session when you screw something up.
- kragen 5y ago
  If it isn't important whether the numbers are real or fake, why show them? To intimidate people with a baseless appearance of mathematical rigor? If so, I'd say the people who point out that they're fake are doing a service to the decisionmakers in the meeting, though it's unsurprising that the presenters don't appreciate it.
  - curiousllama 5y ago
    You've never shipped with known bugs? If someone asked you "If it isn't important whether the software works or not, why ship it?", what would your reaction be? Note: > Directional correctness facilitates the same conversation
    - Jensson 5y ago
      A math error in a spreadsheet can change the entire conclusion to be drawn from the data, it is very different from known bugs users can work around.
      - conductr 5y ago
        But both can only be ascertained in hindsight and knowledge of the issue. Software issues can, and do, also cause a lot of issues with the data that spreadsheets are analyzing so I don’t necessarily see it as different. Also a lot of spreadsheet info is just info. It’s not necessarily being used as a crucial decision criteria. There’s such a wide spectrum of situations it’s hard to be too rigid with this. My software analogy was just pandering to this audience. Probably not a perfect analogy.
    - kragen 5y ago
      I would say that most software doesn't have to be correct to be useful. There are exceptions, like anything to do with cryptography. You might say that the same can be true of calculations; after all, calculations about the contingent universe (as opposed to, say, number theory) are always based on data incorporating some uncertainty. But there are two key differences: ① I don't put up a slide showing all the intermediate values of the variables in my program in order to make an argument for something. Instead, I try to show my chain of reasoning in enough detail to be convincing and to expose any relevant potentially incorrect assumptions or reasoning steps I've made so that others can find my errors, without including trivial or irrelevant details. Including trivial and irrelevant details makes it harder, not easier, to find relevant incorrect assumptions or reasoning steps. That's why I question the motivation of people who include those in their slides: it sounds like they're attempting to make their argument so complex that there are no obvious errors, rather than so simple that there are obviously no errors. ② A 10% uncertainty in an input datum can be traced through the calculation from beginning to end and will often result in a 10% or 21% uncertainty in the result, or even less. (And where that's not the case the sensitivity should be called out.) A calculation that's simply incorrect—for example, treating millions as thousands, or forgetting to divide by the relevant denominator—commonly produces results that are off by multiple orders of magnitude.
kragen 5y ago
Ego. It's a hell of a drug.
singularity2001 5y ago
Conclusion (also from own experience): Mental math in 2022 can be a career boost and significantly increase your appreciation among peers.
curiousllama 5y ago
Happens all the time! Just be careful with this stuff - often, the small mistakes in unimportant stuff are just a junior futzing up something small. If it derails the meeting, you may be ruining some faceless 23 YO's performance review for an otherwise-innocuous bug.
treeman79 5y ago
Was in physics class and the professor put a problem on board to keep the entire class busy for the hour. She had grading or something. I immediately shouted out the answer at first glance. Did that several times.
dotancohen 5y ago
Smugness in meetings is fun, but a comment right here on HN demonstrates that even the presumably-intelligent people browsing HN don't have mental math skills. https://news.ycombinator.com/item?id=30505286 https://news.ycombinator.com/item?id=30505286

conductr 5y ago

It happens. I work in corporate finance and am presenting boring spreadsheet stuff daily. I/My team makes mistakes too and I don’t always catch them before the presentation. Sometimes there’s literally no time it’s like dev on prod without tests. Anyways, what you did is very common. Many people are actively looking for mistakes in the math instead of focusing on the content of the presentation. And will derail my entire meeting when they find one. Just word of warning , don’t be that guy. Usually it’s some unimportant metric (or someone would have noticed) and directional correctness facilitates the same conversation. For example, oh you’re right overtime is running at 10% instead of 8% is usually not a material fact when your department is 30% over budget and some productivity metric is where the businesses focus needs to be (eg. widgets/hour). Instead, bring it to my attention after the fact. Or shoot me a email while in the meeting. Only verbalize it if it’s crucial to the conversation or impacts the decision being made. It’s a bit like expecting bug free software, it’s just impossible. Not saying you handled it wrong perse, most people do this. Just suggesting how I have been on the other side of this and prefer for others to act. I find this more professional as I wouldn’t turn up to your product launch party and just tell everyone in the room about the GitHub issues log. Or, as another example, if I were your manager, I wouldn’t call you out publicly but assume you prefer a private feedback session when you screw something up.

kragen 5y ago
If it isn't important whether the numbers are real or fake, why show them? To intimidate people with a baseless appearance of mathematical rigor? If so, I'd say the people who point out that they're fake are doing a service to the decisionmakers in the meeting, though it's unsurprising that the presenters don't appreciate it.
- curiousllama 5y ago
  You've never shipped with known bugs? If someone asked you "If it isn't important whether the software works or not, why ship it?", what would your reaction be? Note: > Directional correctness facilitates the same conversation
  - Jensson 5y ago
    A math error in a spreadsheet can change the entire conclusion to be drawn from the data, it is very different from known bugs users can work around.
    - conductr 5y ago
      But both can only be ascertained in hindsight and knowledge of the issue. Software issues can, and do, also cause a lot of issues with the data that spreadsheets are analyzing so I don’t necessarily see it as different. Also a lot of spreadsheet info is just info. It’s not necessarily being used as a crucial decision criteria. There’s such a wide spectrum of situations it’s hard to be too rigid with this. My software analogy was just pandering to this audience. Probably not a perfect analogy.
  - kragen 5y ago
    I would say that most software doesn't have to be correct to be useful. There are exceptions, like anything to do with cryptography. You might say that the same can be true of calculations; after all, calculations about the contingent universe (as opposed to, say, number theory) are always based on data incorporating some uncertainty. But there are two key differences: ① I don't put up a slide showing all the intermediate values of the variables in my program in order to make an argument for something. Instead, I try to show my chain of reasoning in enough detail to be convincing and to expose any relevant potentially incorrect assumptions or reasoning steps I've made so that others can find my errors, without including trivial or irrelevant details. Including trivial and irrelevant details makes it harder, not easier, to find relevant incorrect assumptions or reasoning steps. That's why I question the motivation of people who include those in their slides: it sounds like they're attempting to make their argument so complex that there are no obvious errors, rather than so simple that there are obviously no errors. ② A 10% uncertainty in an input datum can be traced through the calculation from beginning to end and will often result in a 10% or 21% uncertainty in the result, or even less. (And where that's not the case the sensitivity should be called out.) A calculation that's simply incorrect—for example, treating millions as thousands, or forgetting to divide by the relevant denominator—commonly produces results that are off by multiple orders of magnitude.

kragen 5y ago

Ego. It's a hell of a drug.

singularity2001 5y ago

Conclusion (also from own experience): Mental math in 2022 can be a career boost and significantly increase your appreciation among peers.

curiousllama 5y ago

Happens all the time! Just be careful with this stuff - often, the small mistakes in unimportant stuff are just a junior futzing up something small. If it derails the meeting, you may be ruining some faceless 23 YO's performance review for an otherwise-innocuous bug.

treeman79 5y ago

Was in physics class and the professor put a problem on board to keep the entire class busy for the hour. She had grading or something. I immediately shouted out the answer at first glance. Did that several times.

dotancohen 5y ago

Smugness in meetings is fun, but a comment right here on HN demonstrates that even the presumably-intelligent people browsing HN don't have mental math skills. https://news.ycombinator.com/item?id=30505286 https://news.ycombinator.com/item?id=30505286