Lexeme.similarity and AnchorText about alibi HOT 4 CLOSED

Hi, thanks for your interest in the library, I look forward to reading the blog post!

1. I've seen the same issue so I'm going to investigate, the warning suggests that we are looking at synonyms for empty vectors, this could be an alibi bug or intended behaviour according to spacy.

2. This is interesting. First I just want to note that this in an example of an empty anchor not a "no anchor" - no anchor would be a partial anchor which did not satisfy the precision threshold no matter how many features were added in. I agree that it looks like the anchor algorithm is picking up on the model's decision to classify as "neither" if apples/oranges are absent, but it's not entirely clear. Can I ask a bit more about what the sentences in the "neither" category look like?

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@mapmeld I believe the cause of 1. is using the small sm model - the spacy docs say that the sm models don't ship with word vectors, I would suggest trying the lg model instead and see if the warnings go away (the quality of synonyms should also improve!).

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@mapmeld I was wrong, we use the md model by default which does have word vectors, the issue arises because there are a lot of lexemes for which the word vector is identically zero. I think we should prune these from the corpus before finding the synonyms. Alternatively we can bump up the default w_prob=-20 to something higher to exclude more words based on rarity.

Edit: I've submitted a PR to fix this #110

Edit2: This is now merged and fixed in v0.2.2

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@mapmeld I will close this issue now as the warnings have been fixed and it's hard to debug the Anchor output without knowing the details of your model, feel free open a new issue if you have more details.

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