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While the Corpus do Português (Web / Dialects) has about one billion words of data, there are much larger web-based corpora. For example, Sketch Engine has a 3.9 billion word corpus of Portuguese. Why not just use a corpus like this instead?

The reason why is that size is not everything. Once the corpus is created, it is annotated for part of speech and lemma (e.g. disse, dizemos, and dirão are all forms of the lemma dizer). While it's easy to create a large corpus from the web for any language nowadays, it's much harder to annotate it correctly and accurately. And without good annotation, the corpus is almost unusable, at least for some purposes.

To correct the corpus, it requires that someone actually know Portuguese. Based on the accuracy of the Sketch Engine, it appears that nobody did. They simply blindly ran the tagger on the corpora and then placed them online, with little or no attempt to fix things. Quick, but not very helpful.

To see what types of problems have resulted from the inaccurate tagging and lemmatization, take a look at the following spreadsheet.
 
   Portuguese lemmas

This spreadsheet shows words starting with s- in the Sketch Engine corpus, which have a frequency of between 1000 and 2000 tokens in the corpus. (In other words, these are relatively frequent words) The spreadsheets group words by lemma and part of speech (noun, verb, adjective). Potential "problem" words are highlighted in yellow and (most problematic) orange.

Taking a look at just the verbs, we find that more than at least 46 of these 68 frequent "verbs" aren't really verbs at all (and these are supposedly common "verbs" -- occurring 1000 times or more). Some of them are forms of Portuguese verbs (saíu, saimos, selecionaram, sabiamos, sorocaba), but they are not actually lemmas (i.e. the entry that one would find in a dictionary). Some of these at least end in an -r, which would suggest that they might be Portuguese verbs in some alternate universe (secalhar, sanduichar, sinistrar, saír, siar, sapar, soccer), but they are not actually words in this universe. And others clearly could never be verbs (at least in Portuguese, the language of the corpus): sensei, sibutramina, simpatica, sm, sabados, semiárido, sobrevivencia, simple, silver, sample.

If we were to go further down the list -- words that occur 100-200 times, for example -- we would find that nearly 90% of all of the entries are problematic. For proof of this, see the data from Spanish, which covers a much larger frequency range, and which used the same (FreeLing) tagger. We've done "probes" of the Portuguese data at different frequency bands, and it is very similar to what that page shows for Spanish. (Since that Spanish data was downloaded, Sketch Engine has blocked people from downloading so much data.) But even with these very frequent "verbs" (which occur 1000-2000 times each in the corpus), the data is extremely messy.

If you're going to create word frequency data or language learning tools like we've done for English, you need to carefully review thousands upon thousands of words -- looking at their context, fixing lemmas and part of speech, etc. And you need to have at least a rudimentary knowledge of the language you're working with. None of this was done for these larger Portuguese corpora and so they are -- as we have mentioned -- almost unusable for many purposes.
 


With our corpus, we are reviewing each and every lemma (for the top 40,000 lemmas in the corpus), to make sure that the lemma and the part of speech are correct. It's a lot of work, and it takes several months to compete. But will such a correction, we believe that we have the only large (> one billion words) and reliable corpus of Portuguese.