5.6 Local Surrogate Models (LIME)
Local interpretable model-agnostic explanations (LIME) (Ribeiro, M.T., Singh, S. and Guestrin, C., 201631) is a method for fitting local, interpretable models that can explain single predictions of any black-box machine learning model. LIME explanations are local surrogate models. Surrogate models are interpretable models (like a linear model or decision tree) that are learned on the predictions of the original black box model. But instead of trying to fit a global surrogate model, LIME focuses on fitting local surrogate models to explain why single predictions were made.
The idea is quite intuitive. First of all, forget about the training data and imagine you only have the black box model where you can input data points and get the models predicted outcome. You can probe the box as often as you want. Your goal is to understand why the machine learning model gave the outcome it produced. LIME tests out what happens to the model’s predictions when you feed variations of your data into the machine learning model. LIME generates a new dataset consisting of perturbed samples and the associated black box model’s predictions. On this dataset LIME then trains an interpretable model weighted by the proximity of the sampled instances to the instance of interest. The interpretable model can basically be anything from this chapter, for example LASSO or a decision tree. The learned model should be a good approximation of the machine learning model locally, but it does not have to be so globally. This kind of accuracy is also called local fidelity.
The recipe for fitting local surrogate models:
- Choose your instance of interest for which you want to have an explanation of its black box prediction.
- Perturb your dataset and get the black box predictions for these new points.
- Weight the new samples by their proximity to the instance of interest.
- Fit a weighted, interpretable model on the dataset with the variations.
- Explain prediction by interpreting the local model.
In the current implementations (R and Python) for example linear regression can be chosen as interpretable surrogate model. Upfront you have to choose \(K\), the number of features that you want to have in your interpretable model. The lower the \(K\), the easier the model is to interpret, higher \(K\) potentially creates models with higher fidelity. There are different methods for how to fit models with exactly \(K\) features. A solid choice is Lasso. A Lasso model with a high regularisation parameter \(\lambda\) yields a model with only the intercept. By refitting the Lasso models with slowly decreasing \(\lambda\), one after each other, the features are getting weight estimates different from zero. When \(K\) features are in the model, you reached the desired number of features. Other strategies are forward or backward selection of features. This means you either start with the full model (=containing all features) or with a model with only the intercept and then testing which feature would create the biggest improvement when added or removed, until a model with \(K\) features is reached. Other interpretable models like decision trees are also possible.
As always, the devil’s in the details. In a high-dimensional space, defining a neighbourhood is not trivial. Distance measures are quite arbitrary and distances in different dimensions (aka features) might not be comparable at all. How big should the neighbourhood be? If it is too small, then there might be no difference in the predictions of the machine learning model at all. LIME currently has a hard coded kernel and kernel width, which define the neighbourhood, and there is no answer how to figure out the best kernel or how to find the optimal width. The other question is: How do you get the variations of the data? This differs depending on the type of data, which can be either text, an image or tabular data. For text and image the solution is turning off and on single words or super-pixels. In the case of tabular data, LIME creates new samples by perturbing each feature individually, by drawing from a normal distribution with mean and standard deviation from the feature.
LIME does a good job in creating selective explanations, which humans prefer. That’s why I see LIME more in applications where the recipient of the explanation is a lay-person or someone with very little time. It is not sufficient for complete causal attributions, so I don’t see LIME in compliance scenarios, where you are legally required to fully explain a prediction. Also for debugging machine learning models it is useful to have all the reasons instead of a few.
5.6.1 LIME for Tabular Data
Tabular data means any data that comes in tables, where each row represents an instance and each column a feature. LIME sampling is not done around the instance of interest, but from the training data’s mass centre, which is problematic. But it increases the likelihood that the outcome for some of the sampled points predictions differ from the data point of interest and that LIME can learn at least some explanation.
It’s best to visually explain how the sampling and local model fitting works:
Let’s look at a concrete example. We go back to the bike rental data and turn the prediction problem into a classification: After accounting for the trend that the bike rental get’s more popular over time we want to know on a given day if the number of rented bikes will be above or below the trend line. You can also interpret ‘above’ as being above the mean bike counts, but adjusted for the trend.
First we train a Random Forest with 100 trees on the classification task. Given seasonal and weather information, on which day will the number of bike rentals be above the trend-free average?
The explanations are created with 2 features. The results of the sparse local linear model that was fitted for two instances with different predicted classes:
It becomes clear from the figure, that it is easier to interpret categorical features than numerical features. A solution is to categorize the numerical features into bins.
5.6.2 LIME for Text
LIME for text differs from LIME for tabular data. Variations of the data are created differently: Starting from the original text, new texts are created by randomly removing words from it. The dataset is represented with binary features for each word. A feature is 1 if the respective word is included and 0 if it was removed.
In this example we classify spam vs. ham of YouTube comments.
The black box model is a decision tree on the document word matrix. Each comment is one document (= one row) and each column is the number of occurrences of a specific word. Decision trees are easy to understand, but in this case the tree is very deep. Also in the place of this tree there could have been a recurrent neural network or a support vector machine that was trained on the embeddings from word2vec. From the remaining comments two were selected for showing the explanations.
Let’s look at two comments of this dataset and the corresponding classes:
|267||PSY is a good guy||0|
|173||For Christmas Song visit my channel! ;)||1|
In the next step we create some variations of the datasets, which are used in a local model. For example some variations of one of the comments:
Each column corresponds to one word in the sentence. Each row is a variation, 1 indicates that the word is part of this variation and 0 indicates that the word has been removed. The corresponding sentence for the first variation is “
Christmas Song visit my ;)”.
And here are the two sentences (one spam, one no spam) with their estimated local weights found by the LIME algorithm:
The word “channel” points to a high probability of spam.
5.6.3 LIME for Images
This section was written by Verena Haunschmid.
LIME for images works differently than for tabular data and text. Intuitively it would not make much sense to perturb single pixels, since a lot more than one pixel contribute to one class. By randomly changing individual pixels, the predictions would probably not change much. Therefore, variations of the samples (i.e. images) are created by performing superpixel segmentation and switching superpixels off. Superpixels are connected pixels with similar colors and can be turned off by replacing each pixel by a user provided color (e.g., a reasonable value would by gray). The user can also provide a probability for turning off a superpixel in each permutation.
Since the computation of image explanations is rather slow, the lime R package contains a precomputed example which we will also use to show the output of the method. Image data is great for visualization, the explanations can be displayed directly on the image samples. Since we can have several predicted labels per image (ordered by probability), we can explain the top
n_labels. For the following image the top 3 predictions were strawberry; candle, taper, wax light; and Granny Smith. The prediction and the explanation in the first case are very reasonable. For the second prediction it is quite interesting to see which part of the image contributed to this class. We could conclude that there were objects labeled as candle, taper, wax light in the training set that looked shiny like the tomato.
Ribeiro, M.T., Singh, S. and Guestrin, C., 2016, August. Why should i trust you?: Explaining the predictions of any classifier. In Proceedings of the 22nd ACM SIGKDD International Conference on Knowledge Discovery and Data Mining (pp. 1135-1144). ACM.↩