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Forecasts need to have error bars

andrewpwheeler.com

21–30 of 165 posts

Re: Forecasts need to have error bars

#21
Uncertainty quantification is a neglected aspect of data science and especially machine learning. Practitioners do not always have the statistical background, and the ML crowd generally has a "predict first and asks questions later" mindset that precludes such niceties.

I always demand error bars.

Re: Forecasts need to have error bars

#22
Yes, please! I was part of an org that ran thousands of online experiments over the course of several years. Having some sort of error bars when comparing the benefit of a new treatment gave a much better understanding.

Some thought it clouded the issue. For example, when a new treatment caused a 1% "improvement", but the confidence interval extended from -10% to 10%, it was clear that the experiment didn't tell us how that metric was affected. This makes the decision feel more arbitrary. But that is exactly the point - the decision is arbitrary in that case, and the confidence interval tells us that, allowing us to focus on other trade-offs involved. If the confidence interval is 0.9% to 1.1%, we know that we can be much more confident in the effect.

A big problem with this is that meaningful error bars can be extremely difficult to come by in some cases. For example, imagine having something like that for every prediction made by an ML model. I would love to have that, but I'm not aware of any reasonable way to achieve it for most types of models. The same goes for online experiments where a complicated experiment design is required because there isn't a way to do random allocation that results in sufficiently independent cohorts.

On a similar note, regularly look at histograms (i.e., statistical distributions) for all important metrics. In one case, we were having speed issues in calls to a large web service. Many calls were completing in < 50 ms, but too many were tripping our 500 ms timeout. At the same time, we had noticed the emergence of two clear peaks in the speed histogram (i.e., it was a multimodal distribution). That caused us to dig a bit deeper and see that the two peaks represented logged-out and logged-in users. That knowledge allowed us to ignore wide swaths of code and spot the speed issues in some recently pushed personalization code that we might not have suspected otherwise.

Re: Forecasts need to have error bars

#23
post #21

Uncertainty quantification is a neglected aspect of data science and especially machine learning. Practitioners do not always have the statistical background, and the ML crowd generally has a "predict first and asks questions later" mindset that precludes such niceties. I always demand error bars.

So is it really science? These are concepts from stats 101. And the reasons and need, and the risks of not having them are very clear. But you have millions being put into models without these pre-requisites, and being sold to people as solutions, and waved away as "if people buy is it's bc it has value". People also pay fraudsters.

Re: Forecasts need to have error bars

#24
Every estimate/prediction/forecast/interpolation/extrapolation should have a confidence/prediction/ or tolerance interval (application dependent) that incorporates the assumptions that the team is putting into the problem.

Re: Forecasts need to have error bars

#25
post #15

Earlier quoted context omitted.

Ok, the 'reverse' of a confidence interval then -- I haven't seen a term for the object I described other than misuse of CI in the way I did. ("Double quantile"?)

You're probably thinking of a predictive interval

It is a very common misconception and one of my technical crusades. I keep fighting, but I think I have lost. Not knowing what the "uncertainty interval" represents (is it, loosely speaking, an expectation about a mean/true value or about the distribution of unobserved values?) could be even more dangerous, in theory, than using no uncertainty interval at all.

I say in theory because, in my experience in the tech industry, with the usual exceptions, uncertainty intervals, for example on a graph, are interpreted by those making decisions as aesthetic components of the graph ("the gray bands look good here") and not as anything even marginally related to a prediction.

Re: Forecasts need to have error bars

#26
post #5

I have, in my life as a web developer, had multiple "academics" urgently demand that i remove error bands, bars, notes about outliers, confidence intervals etc from graphics at the last minute so people are not "confused" Its depressing

It really depends what it is for. If the assessment is that the data is solid enough for certain decisions you might indeed only show a narrow result in order not to waste time and attention. If it is for a scientific discussion then it is different, of course.

Re: Forecasts need to have error bars

#27

I really thought that this was going to be about the weather.

Same, but in a human context, are mundane atmospheric events so far off today that error bars would have any practical value and/or potentially introduce confusion?

Absolutely. 15 years ago I could reasonably trust forecasts regarding whether it’s going to rain in a given location 2 days in advance. Today I can’t trust forecasts about whether it’s raining currently.

Re: Forecasts need to have error bars

#28
post #20
post #5

I have, in my life as a web developer, had multiple "academics" urgently demand that i remove error bands, bars, notes about outliers, confidence intervals etc from graphics at the last minute so people are not "confused" Its depressing

Statistically illiterate people should not be making decisions. I'd take that as a signal to leave.

Statistically speaking, you're in the minority. ;)

Re: Forecasts need to have error bars

#29
post #5

I have, in my life as a web developer, had multiple "academics" urgently demand that i remove error bands, bars, notes about outliers, confidence intervals etc from graphics at the last minute so people are not "confused" Its depressing

Sometimes they do this because the data doesn't entirely support their conclusions. Error bars, noting data outliers etc often make this glaringly apparent.

Re: Forecasts need to have error bars

#30
post #8

Two things I think are interesting here, one discussed by the author and one not. (1) As mentioned at the bottom, forecasting usually should lead to decisionmaking, and when it gets disconnected, it can be unclear what the value is. It sounds like Rosenfield is trying to use forecasting to give added weight to his statistical conclusions about past data, which I agree sounds suspect. (2) it's not clear what the "erro…

> Another is a standard deviation (i.e. you are pretty much predicting the squared difference between your own point forecast and the outcome).

What you probably want is the standard error, because you are not interested in how much your data differ from each other but in how much your data differ from the true population.

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