Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

filiph.github.io

Post reply on HN
Print view View original

151–160 of 170 posts

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#151

Post by jrowen »

kqr wrote: post #78
jrowen wrote: post #76

Earlier quoted context omitted.

This jives with my general reaction to the post, which was that the added complexity and difficulty of reasoning about the ranges actually made me feel less confident in the result of their example calculation. I liked the $50 result, you can tack on a plus or minus range but generally feel like you're about breakeven. On the other hand, "95% sure the real balance will fall into the -$60 to +$220 range" feels like it…

It is producing a useful number. As more truly independent terms are added, error grows with the square root while the point estimation grows linearly. In the aggregate, the error makes up less of the point estimation. This is the reason Fermi estimation works. You can test people on it, and almost universally they get more accurate with this method. If you got less certain of the result in the example, that's probab…

Read a bit on Fermi estimation, I'm not quite sure exactly what the "method" is in contrast to a less accurate method, it's basically just getting people to think in terms of dimensional analysis? This passage from the Wikipedia is interesting:

By contrast, precise calculations can be extremely complex but with the expectation that the answer they produce is correct. The far larger number of factors and operations involved can obscure a very significant error, either in mathematical process or in the assumptions the equation is based on, but the result may still be assumed to be right because it has been derived from a precise formula that is expected to yield good results.

So the strength of it is in keeping it simple and not trying to get too fancy, with the understanding that it's just a ballpark/sanity check. I still feel like the Drake equation in particular has too many terms for which we don't have enough sample data to produce a reasonable guess. But I think this is generally understood and it's seen as more of a thought experiment.

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#153

Post by baq »

duckmysick wrote: post #143
peeters wrote: post #139

Earlier quoted context omitted.

So the context of the quiz is software estimation, where I assume it's an intentional parable of estimating something you haven't seen before. It's trying to demonstrate that your "5-7 days" estimate probably represents far more certainty than you intended. For some of these, your answer could span orders of magnitude. E.g. my answer for the heaviest blue whale would probably be 5-500 tons because I don't have a good…

I don't know, an estimate spanning three orders of magnitude doesn't seem useful. To continue your example of 5-7 days, it would turn into an estimate of 5-700 days. So somewhere between a week or two years. And fair enough, whatever you're estimating will land somewhere in between. But how do I proceed from there with actual planning or budget?

Your question is akin to asking ‘how do I make the tail to wag the dog?’

Your budget should be allocated for say 80% confidence (which the tool helpfully provides behind a switch) and your stakeholders must be on board with this. It shouldn’t be too hard to do since everyone has some experience with missed engineering deadlines. (Bezos would probably say 70% or even less.)

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#154

Post by NunoSempere »

joshlemer wrote: post #142
NunoSempere wrote: post #58

Earlier quoted context omitted.

Another tool in this spirit is https://carlo.app/ >, which allows you to do this kind of calculation on google sheets.

Their pricing is absolutely out of this world though. Their BASIC plan is $2990 USD per year, the pro plan is $9990/year. https://carlo.app/pricing

They have a free tier as well, just with fewer samples, and aren't in the zero marginal cost regime

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#155

Post by dawnofdusk »

roughly wrote: post #44

I like this! In the grand HN tradition of being triggered by a word in the post and going off on a not-quite-but-basically-totally-tangential rant: There’s (at least) three areas here that are footguns with these kinds of calculations: 1) 95% is usually a lot wider than people think - people take 95% as “I’m pretty sure it’s this,” whereas it’s really closer to “it’d be really surprising if it were not this” - by and…

>rent is very likely to be correlated with food costs - so, if rent is high, food costs are also likely to be high

Not sure I agree with this. It's reasonable to have a model where the mean rent may be correlated with the mean food cost, but given those two parameters we can model the fluctuations about the mean as uncorrelated. In any case at the point when you want to consider something like this you need to do proper Bayesian statistics anyways.

>In general normal distributions are rarer than people think - they tend to require some kind of constraining factor on the values to enforce.

I don't know where you're getting this from. One needs uncorrelated errors, but this isn't a "constraint" or "negative feedback".

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#157

Post by blackbear_ »

SamBam wrote: post #61
NunoSempere wrote: post #57

Earlier quoted context omitted.

I'm guessing this is not an error. If you divide 1/normal(0,1), the full distribution would range from -inf to inf, but the 95% output doesn't have to.

I don't quite understand, probably because my math isn't good enough. If you're treating -1~1 as a normal distribution, then it's centered on 0. If you're working out the answer using a Monte Carlo simulation, then you're going to be testing out different values from that distribution, right? And aren't you going to be more likely to test values closer to 0? So surely the most likely outputs should be far from 0, rig…

If X is normal and centered around 0, then the average of 1/X does not exist (math speak for "is infinity" in this case). In these cases Monte Carlo simulations are not reliable because they give high variance estimates (math speak for "the histogram varies run by run").

The actual distribution of 1/X is fairly interesting, see https://en.m.wikipedia.org/wiki/Inverse_distribution#Recipro...

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#158

Post by blackbear_ »

lswainemoore wrote: post #71
pyfon wrote: post #64

Earlier quoted context omitted.

Only 1 percent of values would end up being 100+ on a uniform distribution. For normal it is higher but maybe not much more so.

That may be true, but if you look at the distribution it puts out for this, it definitely smells funny. It looks like a very steep normal distribution, centered at 0 (ish). Seems like it should have two peaks? But maybe those are just getting compressed into one because of resolution of buckets?

It does indeed have two peaks: https://en.m.wikipedia.org/wiki/Inverse_distribution#Recipro...

As the mean of this distribution does not exist (it's "infinite") the Monte Carlo estimates aren't reliable

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#159

Post by pests »

kqr wrote: post #104
pests wrote: post #86

Earlier quoted context omitted.

> People are default overconfident with their estimated error bars. You say this but yet roughly in a top level comment mentions people keep their error bars too close.

Sorry, my comment was phrased confusingly. Being overconfident with error bars means placing them too close to the point estimation, i.e. the error bars are too narrow.

Ah right thanks, I read that backwards.

Re: Show HN: Unsure Calculator – back-of-a-napkin probabilistic calculator

#160

Post by croisillon »

constantcrying wrote: post #24
croisillon wrote: post #19

i like it and i skimmed the post but i don't understand why the default example 100 / 4~6 has a median of 20? there is no way of knowing why the range is between 4 and 6

The chance of 4~6 being less than 5 is 50%, the chance of it being greater is also 50%. The median of 100/4~6 has to be 100/5. >there is no way of knowing why the range is between 4 and 6 ??? There is. It is the ~ symbol.

mmh ok thanks, i guess i need extra maths training ;)

i didn't mean knowing _that_ the range is between 4 and 6 but _why_, i thought the weighing would be explained by the reasoning, like: "we divide a €100 bill between possibly 4, rather 5 and most probably not 6 persons"

Post reply on HN