Not being able to say why you see a 2 doesn’t excuse your uninterpretable model

This is Jessica, but today I’m blogging about a blog post on interpretable machine learning that co-blogger Keith wrote for his day job and shared with me. I agree with multiple observations he makes. Some highlights:

The often suggested simple remedy for this unmanageable complexity is just finding ways to explain these black box models; however, those explanations can sometimes miss key information. In turn, rather than being directly connected with what is going on in the black box model, they result in being “stories” for getting concordant predictions. Given that concordance is not perfect, they can result in very misleading outcomes for many situations.

I blogged a bit about this before, giving examples like inconsistency in explanations for the same inputs and outputs. Thinking about these complications, I’m reminded of a talk by Chris Olah that I saw back in 2018, where he talked about how feature visualizations of activiations that fire for different image inputs to a deep neural net allow us to seriously consider what’s going on inside, in a way that makes them analogous to the discovery of the microscope opening up a whole new world of microorganisms. I wonder if this idea has lost favor given that sometimes these explanations don’t behave the way we would hope.  

I can also buy that not-quite-correct explanations can cause problems downstream since I see it in the human context. The other day I had to ask a collaborator to try to refrain from providing explanations instead of methodological details for unexpected analysis results, since when delivered passionately the explanations could seem good enough that I wouldn’t question them initially, only to later realize we wasted time when there was a better explanation. Plus when every unexpected result comes with explanation, skepticism with the explanation can make it feel like you’re undercutting everything a person says, even if you want to encourage discussion of these things. 

While we need to accept what we cannot understand, we should never overlook the advantages of what we can understand. For example, we may never fully understand the physical world. Nor how people think, interact, create and or decide. In ML, Geoffrey Hinton’s 2018 YouTube drew attention to the fact that people are unable to explain exactly how they decide in general if something is the digit 2 or not. This fact was originally pointed out, a while ago, by Herbert Simon, and has not been seriously disputed (Erickson and Simon, 1980). However, prediction models are just abstractions and we can understand the abstractions created to represent that reality, which is complex and often beyond our direct access. So not being able to understand people is not a valid reason to dismiss desires to understand prediction models.

In essence, abstractions are diagrams or symbols that can be manipulated, in error-free ways, to discern their implications. Usually referred to as models or assumptions, they are deductive and hence can be understood in and of themselves for simply what they imply. That is, until they become too complex. For instance, triangles on the plane are understood by most, while triangles on the sphere are understood by less. Reality may always be too complex, but models that adequately represent reality for some purpose need not be. Triangles on the plane are for navigation of short distances while on the sphere, for long distances. Emphatically, it is the abstract model that is understood not necessarily the reality it attempts to represent.

I’m not sure I totally grasp the distinction Keith is trying to get at here. To me the above passage implies we should be careful about assuming that some aspects of reality are too complex to explain. But given the part about concordances being misleading above, it seems applying this recursively can lead to problems: when the deep model is the complex thing we want to explain, we have to be careful isolating what we think are simpler units of abstractions to capture what it’s doing. For instance, a node in a neural network is a relatively simple abstraction (i.e., a linear regression wrapped in a non-linear activation function), but is thinking at that level of abstraction as a means of trying to understand the much more complex behavior of the network as a whole useful? Maybe Keith is trying to motivate considering interpretability in your choice of model, which he talks about later. 

Related to people not being able to say how they recognize a 2, one thing that people can potentially do is point to the processor they think is responsible; e.g., I can’t describe why it’s a 2 succintly based on low level properties like edge detection but maybe I could say something higher level like, ‘I would guess it’s something like visual word form memory.’ It’s not a complete explanation, but it seems that sort of meta statement could potentially be useful since the first step to debugging is to figure out where to start looking.

[A] persistent misconception has arisen in ML that models for accurate prediction usually need to be complex. To build upon previous examples, there remains some application areas where simple models have yet to achieve accuracy comparable to black box models. On the other hand, simple models continue to predict as accurately as any state of the art black box model and thus, the question, as noted in the 2019 article by Rudin and Radin, is: “Why Are We Using Black Box Models in AI When We Don’t Need To?”

The referenced paper describes how the authors entered a NeurIPS competition on explainability, but then realized they didn’t need a black box at all to do the task, they could just use one of many simpler, interpretable models. Oops. Some of the interpretability work coming out of ML does seem like what you get when complexity enthusiasts excitedly latch onto new problem that can motivate more of what they’re good at (e.g., optimization), without necessarily questioning the premise.   

Interpretable models are far more trustworthy in that they can be more readily discerned where and when they should be trusted or not and in what ways. But, how can one do this without understanding how the model works, especially for a model that is patently not trustworthy? This is especially important in cases where the underlying distribution of data changes, where it is critical to trouble shoot and modify without delays, as noted in the 2020 article by Hamamoto et al. It is arguably much more difficult to remain successful in the ML full life cycle with black box models than with interpretable models.

Agreed; debugging calls for some degree of interpretability. And often the more people you can get helping debug something, the more likely you are to find the problem. 

There is increasing understanding based on considering numerous possible prediction models in a given prediction task. The not-too-unusual observation of simple models performing well for tabular data (a collection of variables, each of which has meaning on its own) was noted over 20 years ago and was labeled the “Rashomon effect” (Breiman, 2001). Breiman posited the possibility of a large Rashomon set in many applications; that is, a multitude of models with approximately the same minimum error rate. A simple check for this is to fit a number of different ML models to the same data set. If many of these are as accurate as the most accurate (within the margin of error), then many other untried models might also be. A recent study (Semenova et al., 2019), now supports running a set of different (mostly black box) ML models to determine their relative accuracy on a given data set to predict the existence of a simple accurate interpretable model—that is, a way to quickly identify applications where it is a good bet that accurate interpretable prediction model can be developed.

I like the idea of trying to estimate how many different ways there are to achieve good accuracy on some inference problem. I’m reminded of a paper I recently read which does basically the inverse – generate a bunch of hypothetical datasets and see how well a model intended to explain human behavior does across them, to understand when you just have a very flexible model versus when it’s actually providing some insight into behavior. 

The full data science and life-cycle process likely is different when using interpretable models. More input is needed from domain experts to produce an interpretable model that make sense to them. This should be seen as an advantage. For instance, it is not too unusual at a given stage to find numerous equally interpretable and accurate models. To the data scientist, there may seem little to guide the choice between these. But, when shown to domain experts, they may easily discern opportunities to improve constraints as well as indications of which ones are less likely to generalize well. All equally interpretable and accurate models are not equal in the eyes of domain experts.

I definitely agree with this and other comments Keith makes about the need to consider interpretability early in the process. I was involved in a paper a few years ago where my co-authors had interviewed a bunch of machine learning developers about interpretability. One of the more surprising things we found was that in contrast to ML lit implying that interpretability can be applied post model development, it was seen by many of the developers as a more holistic thing related to how much others in their organization trusted their work at all, and consequently many thought about from the beginning of model development. 

There is now a vast and confusing literature, which conflates interpretability and explainability. In this brief blog, the degree of interpretability is taken simply as how easily the user can grasp the connection between input data and what the ML model would predict. Erasmus et al. (2020) provide a more general and philosophical view. Rudin et al. (2021) avoid trying to provide an exhaustive definition by instead providing general guiding principles to help readers avoid common, but problematic ways of thinking about interpretability. On the other hand, the term “explainability” often refers to post hoc attempts to explain a black box by using simpler ‘understudy’ models that predict the black box predictions.

I’ve always found the simple definition of interpretability as ability to simulate what a model will predict interesting. At one point I was thinking about how if interpretability is mainly aimed at building trust in model predictions, maybe a “deeper”  proxy for trust could be called internalizability, which is where the person (after using the model) is simulating the model but they don’t know it. 

17 thoughts on “Not being able to say why you see a 2 doesn’t excuse your uninterpretable model

  1. I’m reminded of discussions about sports training. I can be hard for us to describe what we do when we swing a bat or throw a ball; indeed, our descriptions of our own actions may be highly inaccurate, as we can see from going back and watching a video. Usually we think that if we want to improve, it’s a good idea to figure out what we’re currently doing.

    Relatedly, when I teach a skill (whether in statistics or sports or whatever), I become much more aware of my current practice.

  2. This post reminds me of the AI startup where I used to work.
    There was a cult of deep learning. Complex models exerted a powerful fascination.
    The longer the name the better.
    GAN based meta X learning of hyperbolic latent space would soon take over the world, then covid hit, then half of the company was laid off…

    • I’m currently working on evaluating the fire risk of various parts of electricity infrastructure — at the level of individual electric poles and stretches of wire — based on historical data on fires and electric outages. The culture and practices of the team are very much aligned with machine learning; the idea of hand-defining and hand-tuning a model is pretty much a non-starter. So I started with random forest models and then gradient-boosted forest models. But just for laffs I put the same variables into a simple logistic regression model…and it performs just as well. Not better, but just as well. Much easier to interpret and to explain, and equally good empirically. And with some careful selection of interaction terms it can probably improve a bit. I’m going to argue for making the logistic regression model the official one, but I’m expecting some pushback.

      • I’m currently working on evaluating the fire risk of various parts of electricity infrastructure — at the level of individual electric poles and stretches of wire — based on historical data on fires and electric outages. The culture and practices of the team are very much aligned with machine learning

        This is frankly terrifying to me. Fire risk is exactly where rigorous uncertainty quantification seems important—poorly thought out normality assumptions implicit in mean squared loss functions should be a non starter.

      • you have sufficient historical fire data at the level of individual poles? Do you incorporate information about the local topographic, forest and/or typical weather conditions (i.e., topographic features can create different wind conditions; S or SE facing slopes drier), or is that done by a different group or something? Seems like it would be relevant.

      • I’m reminded of the WWII warplane joke. The right place to put the additional armoring is the place where you don’t see any holes in returning planes, since planes that were hit in those places went down and didn’t return. (The holes you see in returning planes are holes that didn’t knock out the plane. So those holes aren’t representative of places that you don’t want to get shot in.)

        The story here would be: places that already have had fires are places that don’t have a lot of stuff (e.g. dried dead underbrush) that causes fires, since the fires already burned that stuff. But the places where fires are likely to occur are places where fires haven’t occurred (and this still have a danger of fires).

        How does ML do on this sort of problem?

  3. > Perhaps and in the end it may boil down to the fact that if simple works, then why make things more complex.

    I’d just add to this testing. Things are easier to understand if they’re tested well. If you can test well enough, then there’s less of a need to understand what’s going on (cuz you’ve defined what’s going on by the tests or whatever).

    I don’t think testing these sorts of things would necessarily be easy, even if you can use off the shelf neural network components or somehow don’t have to deal with most of the training difficulties. It might be easier than explaining the neural network, and you’d need to do the testing either way.

  4. As a child, movies led me to believe I would frequently run into dangerous quicksand and lava, so having a grappling hook handy was essential. In the same way, school led me to believe I would constantly be classifying lots of unstructured data like bitmaps and so I had to have a deep neural network handy.

    Great post–reminds me of this blog by Wolfram (curse his name)

  5. The cover of my book on information quality is “42”. I gave several talks titled “42” asking the audience what it means. Having 42 near 38 and 43 makes is an OK number. Having it next to 0.00000000000000000000000000000000000000007 makes is huge. However, near 2589000000000000000000, makes is small.

    The trick is the context. If you add a story on where the number comes from, the number becomes data.

    Hinton’s YouTube on “2” is similar. Computer scientists think differently from statisticians. He want a definition of “2” without considering context.

    Another point is the possibility to explain something by demarcation of a Boundary of Meaning. You can explain what something is, and what it is not. I called this generalization of findings.

    • Sameness: two things cannot be completely the same in all respects; else they would be one; and not two. Difference: two things cannot be entirely different in every respect; for even then they would have that in common – at least; that relation of difference. What is a thing and what is a collection of things: is it not apparent that the distinction is – properly speaking – really a phenomenon of the attention?

    • Don’t think context was overlooked.

      The inaccurate reports found by other research [e.g. asking subjects how they decided] are shown to result from requesting information that was never directly heeded, thus forcing S[ubject]s to _infer_ rather than _remember_ their mental processes.

      Andrew provided a physical example of this

      • I don’t doubt that people sometimes make things up when asked to verbalize about information they never paid attention to. I guess that’s why people who use think-aloud protocol have to be so careful about not asking leading questions. I have found it useful though in experimental settings to ask people to describe their strategies as they do some information task. It’s always a risk that they aren’t accurate, so it’s never our main analysis, but often in experiments on decision making with some interface, given the amount and kind of data we can collect, it’s not possible to specify a model that can differentiate between multiple similar strategies a person might be using. At least not without designing some series of task with the express purpose of doing that.

    • Would not suggest this line of research won’t get anywhere, but given the intuitive explanation some concerns immediately arise.

      “The primary intuition behind our approach is to identify examples that the simple model will most likely fail on, i.e. identify truly hard examples. We then want to inform the simple model to ignore these examples.”

      Hopefully the easiness was determined on hold out samples (i.e. not just kicking out data points with largest observed prediction errors).

      Simple is not necessarily interpretable except maybe in extremely simplicity and method preferences can largely be unconnected with interpretability (e.g. preference for Cox proportional regression in survival analysis).

      The non-easy samples maybe the more critical ones to get even somewhat close prediction for (e.g. highly infectious variants).

      Given the prediction is either a number or label “too hard”, the “too hard” predictions are left as even blacker box output than the deep neural net predictions.

      However a prediction model taken as just a model is just math and in math, how it was happened upon, simply does not matter. If an accurate interpretable is happened upon – it is what it is.

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