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Anchors

Overview

The anchor algorithm is based on the Anchors: High-Precision Model-Agnostic Explanations paper by Ribeiro et al. and builds on the open source code from the paper’s first author.

The algorithm provides model-agnostic (black box) and human interpretable explanations suitable for classification models applied to images, text and tabular data. The idea behind anchors is to explain the behaviour of complex models with high-precision rules called anchors. These anchors are locally sufficient conditions to ensure a certain prediction with a high degree of confidence.

Anchors address a key shortcoming of local explanation methods like LIME which proxy the local behaviour of the model in a linear way. It is however unclear to what extent the explanation holds up in the region around the instance to be explained, since both the model and data can exhibit non-linear behaviour in the neighborhood of the instance. This approach can easily lead to overconfidence in the explanation and misleading conclusions on unseen but similar instances. The anchor algorithm tackles this issue by incorporating coverage, the region where the explanation applies, into the optimization problem. A simple example from sentiment classification illustrates this (Figure 1). Dependent on the sentence, the occurrence of the word not is interpreted as positive or negative for the sentiment by LIME. It is clear that the explanation using not is very local. Anchors however aim to maximize the coverage, and require not to occur together with good or bad to ensure respectively negative or positive sentiment.

LIMEsentiment

Ribeiro et al., Anchors: High-Precision Model-Agnostic Explanations, 2018

As highlighted by the above example, an anchor explanation consists of if-then rules, called the anchors, which sufficiently guarantee the explanation locally and try to maximize the area for which the explanation holds. This means that as long as the anchor holds, the prediction should remain the same regardless of the values of the features not present in the anchor. Going back to the sentiment example: as long as not good is present, the sentiment is negative, regardless of the other words in the movie review.

Text

For text classification, an interpretable anchor consists of the words that need to be present to ensure a prediction, regardless of the other words in the input. The words that are not present in a candidate anchor can be sampled in 3 ways:

  • Replace word token by UNK token.

  • Replace word token by sampled token from a corpus with the same POS tag and probability proportional to the similarity in the embedding space. By sampling similar words, we keep more context than simply using the UNK token.

  • Replace word tokens with sampled tokens according to the masked language model probability distribution. The tokens can be sampled in parallel, independent of one another, or sequentially(autoregressive), conditioned on the previously generated tokens.

Tabular Data

Anchors are also suitable for tabular data with both categorical and continuous features. The continuous features are discretized into quantiles (e.g. deciles), so they become more interpretable. The features in a candidate anchor are kept constant (same category or bin for discretized features) while we sample the other features from a training set. As a result, anchors for tabular data need access to training data. Let’s illustrate this with an example. Say we want to predict whether a person makes less or more than £50,000 per year based on the person’s characteristics including age (continuous variable) and marital status (categorical variable). The following would then be a potential anchor: Hugo makes more than £50,000 because he is married and his age is between 35 and 45 years.

Images

Similar to LIME, images are first segmented into superpixels, maintaining local image structure. The interpretable representation then consists of the presence or absence of each superpixel in the anchor. It is crucial to generate meaningful superpixels in order to arrive at interpretable explanations. The algorithm supports a number of standard image segmentation algorithms (felzenszwalb, slic and quickshift) and allows the user to provide a custom segmentation function.

The superpixels not present in a candidate anchor can be masked in 2 ways:

  • Take the average value of that superpixel.

  • Use the pixel values of a superimposed picture over the masked superpixels.

anchorimage

Ribeiro et al., Anchors: High-Precision Model-Agnostic Explanations, 2018

Efficiently Computing Anchors

The anchor needs to return the same prediction as the original instance with a minimal confidence of e.g. 95%. If multiple candidate anchors satisfy this constraint, we go with the anchor that has the largest coverage. Because the number of potential anchors is exponential in the feature space, we need a faster approximate solution.

The anchors are constructed bottom-up in combination with beam search. We start with an empty rule or anchor, and incrementally add an if-then rule in each iteration until the minimal confidence constraint is satisfied. If multiple valid anchors are found, the one with the largest coverage is returned.

In order to select the best candidate anchors for the beam width efficiently during each iteration, we formulate the problem as a pure exploration multi-armed bandit problem. This limits the number of model prediction calls which can be a computational bottleneck.

For more details, we refer the reader to the original paper.

Usage

While each data type has specific requirements to initialize the explainer and return explanations, the underlying algorithm to construct the anchors is the same.

In order to efficiently generate anchors, the following hyperparameters need to be set to sensible values when calling the explain method:

  • threshold: the previously discussed minimal confidence level. threshold defines the minimum fraction of samples for a candidate anchor that need to lead to the same prediction as the original instance. A higher value gives more confidence in the anchor, but also leads to more computation time. The default value is 0.95.

  • tau: determines when we assume convergence for the multi-armed bandit. A bigger value for tau means faster convergence but also looser anchor conditions. By default equal to 0.15.

  • beam_size: the size of the beam width. A bigger beam width can lead to a better overall anchor at the expense of more computation time.

  • batch_size: the batch size used for sampling. A bigger batch size gives more confidence in the anchor, again at the expense of computation time since it involves more model prediction calls. The default value is 100.

  • coverage_samples: number of samples used to compute the coverage of the anchor. By default set to 10000.

Text

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Predictor

Since the explainer works on black-box models, only access to a predict function is needed. The model below is a simple logistic regression trained on movie reviews with negative or positive sentiment and pre-processed with a CountVectorizer:

predict_fn = lambda x: clf.predict(vectorizer.transform(x))

Simple sampling strategies

AnchorText provides two simple sampling strategies: unknown and similarity. Randomly chosen words, except those in queried anchor, are replaced by the UNK token for the unknown strategy, and by similar words with the same part of speech of tag for the similarity strategy.

To perform text tokenization, pos-tagging, compute word similarity, etc., we use spaCy. The spaCy model can be loaded as follows:

import spacy
from alibi.utils import spacy_model

model = 'en_core_web_md'
spacy_model(model=model)
nlp = spacy.load(model)

If we choose to replace words with the UNK token, we define the explainer as follows:

explainer = AnchorText(predictor=predict_fn, sampling_strategy='unknown', nlp=nlp)

Likewise, if we choose to sample similar words from a corpus, we define the explainer as follows:

explainer = AnchorText(predictor=predict_fn, sampling_strategy='similarity', nlp=nlp)

Language model

AnchorText provides the option to define the perturbation distribution through a language_model sampling strategy. In this case, randomly chosen words, except those in the queried anchor, are replaced by words sampled according to the language model’s predictions. We provide support for three transformer based language models: DistilbertBaseUncased, BertBaseUncased, and RobertaBase.

A language model can be loaded as follows:

language_model = DistilbertBaseUncased()

Then we can initialize the explainer as follows:

explainer = AnchorText(predictor=predict_fn, sampling_strategy="language_model",
                       language_model=language_model)

Sampling parameters

Parameters specific to each sampling strategy can be passed to the constructor via kwargs. For example:

  • If sampling_strategy="unknown" we can initialize the explainer as follows:

explainer = AnchorText(
    predictor=predict_fn,
    sampling_strategy='unknown',       # replace a word by UNK token
    nlp=nlp,                           # spacy object
    sample_proba=0.5,                  # probability of a word to be replaced by UNK token
)
  • If sampling_strategy="similarity" we can initialize the explainer as follows:

explainer = AnchorText(
    predictor=predict_fn,
    sampling_strategy='similarity',    # replace a word by similar words
    nlp=nlp,                           # spacy object
    sample_proba=0.5,                  # probability of a word to be replaced by as similar word
    use_proba=True,                    # sample according to the similarity distribution
    top_n=20,                          # consider only top 20 most similar words
    temperature=0.2                    # higher temperature implies more randomness when sampling
)
  • Or if sampling_strategy="language_model", the explainer can be defined as:

explainer = AnchorText(
    predictor=predict_fn,
    sampling_strategy="language_model",   # use language model to predict the masked words
    language_model=language_model,        # language model to be used
    filling="parallel",                   # just one pass through the transformer
    sample_proba=0.5,                     # probability of masking and replacing a word according to the LM
    frac_mask_templates=0.1,              # fraction of masking templates
    use_proba=True,                       # use words distribution when sampling (if false sample uniform)
    top_n=50,                             # consider the fist 50 most likely words
    temperature=0.2,                      # higher temperature implies more randomness when sampling
    stopwords=['and', 'a', 'but'],        # those words will not be masked/disturbed
    punctuation=string.punctuation,       # punctuation tokens contained here will not be masked/disturbed
    sample_punctuation=False,             # if False tokens included in `punctuation` will not be sampled
    batch_size_lm=32                      # batch size used for the language model
)

Words outside of the candidate anchor can be replaced by UNK token, similar words, or masked out and replaced by the most likely words according to language model prediction, with a probability equal to sample_proba. We can sample the top n most similar words or the top n most likely language model predictions by setting the top_n parameter. We can put more weight on similar or most likely words by decreasing the temperature argument. It is also possible to sample words from the corpus proportional to the word similarity with the ground truth word or according to the language model’s conditional probability distribution by setting use_proba to True. Furthermore, we can avoid masking specific words by including them in the stopwords list.

Working with transformers can be computationally and memory-wise expensive. For sampling_strategy="language_model" we provide two methods to predict the masked words: filling="parallel" and filling="autoregressive".

If filling="parallel", we perform a single forward pass through the transformer. After obtaining the probability distribution of the masked words, each word is sampled independently of the others.

If filling="autoregressive", we perform multiple forward passes through the transformer and generate the words one at a time. Thus, the masked words will be conditioned on the previous ones. Note that this filling method is computationally expensive.

To further decrease the explanation runtime, for sampling_strategy="language_model", filling="parallel", we provide a secondary functionality through the frac_mask_templates. Behind the scenes, the anchor algorithm is constantly requesting samples to query the predictor. Thus, we need to generate what we call mask templates, which are sentences containing words outside the candidate anchors replaced by the <MASK> token. The frac_mask_templates controls the fraction of mask templates to be generated. For example, if we need to generate 100 samples and the frac_mask_templates=0.1, we will generate only 10 mask templates. Those 10 templates are then passed to the language model to predict the masked words. Having the distribution of each word in each mask template, we can generate 100 samples as requested. Note that instead of passing 100 masked sentences through the language model (which is expensive), we only pass 10 sentences. Although this can increase the speed considerably, it can also decrease the diversity of the samples. The maximum batch size used in a forward pass through the language model can be specified by setting batch_size_lm.

When sampling_strategy="language_model", we can specify the punctuation considered by the sampling algorithm. Any token composed only from characters in the punctuation string, will not be perturbed (we call those punctuation tokens). Furthermore, we can decide whether to sample punctuation tokens by setting the sample_punctuation parameter. If sample_punctuation=False, then punctuation tokens will not be sampled.

Explanation

Let’s define the instance we want to explain and verify that the sentiment prediction on the original instance is positive:

text = 'This is a good book .'
class_names = ['negative', 'positive']
pred = class_names[predict_fn([text])[0]]

Now we can explain the instance:

explanation = explainer.explain(text, threshold=0.95)

The explain method returns an Explanation object with the following attributes:

  • anchor: a list of words in the anchor.

  • precision: the fraction of times the sampled instances where the anchor holds yields the same prediction as the original instance. The precision will always be \(\geq\) threshold for a valid anchor.

  • coverage: the fraction of sampled instances the anchor applies to.

The raw attribute is a dictionary which also contains example instances where the anchor holds and the prediction is the same as on the original instance, as well as examples where the anchor holds but the prediction changed to give the user a sense of where the anchor fails. raw also stores information on the anchor, precision and coverage of partial anchors. This allows the user to track the improvement in for instance the precision as more features (words in the case of text) are added to the anchor.

Tabular Data

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Initialization and fit

To initialize the explainer, we provide a predict function, a list with the feature names to make the anchors easy to understand as well as an optional mapping from the encoded categorical features to a description of the category. An example for categorical_names would be

category_map = {0: ["married", "divorced"], 3: ["high school diploma", "master's degree"]}

Each key in category_map refers to the column index in the input for the relevant categorical variable, while the values are lists with the options for each categorical variable. To make it easy, we provide a utility function gen_category_map to generate this map automatically from a Pandas dataframe:

from alibi.utils import gen_category_map
category_map = gen_category_map(df)

Then initialize the explainer:

predict_fn = lambda x: clf.predict(preprocessor.transform(x))
explainer = AnchorTabular(predict_fn, feature_names, categorical_names=category_map)

The implementation supports one-hot encoding representation of the cateforical features by setting ohe=True. The feature_names and categorical_names(category_map) remain unchanged. The prediction function predict_fn should expect as input datapoints with one-hot encoded categorical features. To initialize the explainer with the one-hot encoding support:

explainer = AnchorTabular(predict_fn, feature_names, categorical_names=category_map, ohe=True)

Tabular data requires a fit step to map the ordinal features into quantiles and therefore needs access to a representative set of the training data. disc_perc is a list with percentiles used for binning:

explainer.fit(X_train, disc_perc=[25, 50, 75])

Note that if one-hot encoding support is enabled (ohe=True), the fit calls expect the data to be one-hot encoded.

Explanation

Let’s check the prediction of the model on the original instance and explain:

class_names = ['<=50K', '>50K']
pred = class_names[explainer.predict_fn(X)[0]]
explanation = explainer.explain(X, threshold=0.95)

The returned Explanation object contains the same attributes as the text explainer, so you could explain a prediction as follows:

Prediction:  <=50K
Anchor: Marital Status = Never-Married AND Relationship = Own-child
Precision: 1.00
Coverage: 0.13

Note that if one-hot encoding support is enabled (ohe=True), the explain calls expect the data to be one-hot encode.

Images

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Initialization

Besides the predict function, we also need to specify either a built in or custom superpixel segmentation function. The built in methods are felzenszwalb, slic and quickshift. It is important to create sensible superpixels in order to speed up convergence and generate interpretable explanations. Tuning the hyperparameters of the segmentation method is recommended.

explainer = AnchorImage(predict_fn, image_shape, segmentation_fn='slic',
                        segmentation_kwargs={'n_segments': 15, 'compactness': 20, 'sigma': .5},
                        images_background=None)

Example of superpixels generated for the Persian cat picture using the slic method:

persiancat persiancatsegm

The following function would be an example of a custom segmentation function dividing the image into rectangles.

def superpixel(image, size=(4, 7)):
    segments = np.zeros([image.shape[0], image.shape[1]])
    row_idx, col_idx = np.where(segments == 0)
    for i, j in zip(row_idx, col_idx):
        segments[i, j] = int((image.shape[1]/size[1]) * (i//size[0]) + j//size[1])
    return segments

The images_background parameter allows the user to provide images used to superimpose on the masked superpixels, not present in the candidate anchor, instead of taking the average value of the masked superpixel. The superimposed images need to have the same shape as the explained instance.

Explanation

We can then explain the instance in the usual way:

explanation = explainer.explain(image, p_sample=.5)

p_sample determines the fraction of superpixels that are either changed to the average superpixel value or that are superimposed.

The Explanation object again contains information about the anchor’s precision, coverage and examples where the anchor does or does not hold. On top of that, it also contains a masked image with only the anchor superpixels visible under the anchor attribute (see image below) as well as the image’s superpixels under segments.

persiancatanchor

Examples

Image

Anchor explanations for ImageNet

Anchor explanations for fashion MNIST

Tabular Data

Anchor explanations on the Iris dataset

Anchor explanations for income prediction

Text

Anchor explanations for movie sentiment