Stemming
Cutting words to a common root, such as 'downgrad' for downgrade and downgraded, so variants count as one token. Cruder than lemmatisation, which maps each to a dictionary word.
Where you will meet it
Stemming appears in 2 learning outcomes on the CFA syllabus. A few of them:
- Explain techniques for extracting, choosing and engineering features from text data
“Tokenisation splits it into words; normalisation lowercases, removes stop words, and applies stemming or lemmatisation.”
- Explain how text-based data is prepared, wrangled and explored for use in financial forecasting
“Preprocessing then tokenises and normalises: lowercasing, removing stop words, and stemming (cutting to a root such as "downgrad") or lemmatisation (mapping to a dictionary word).”
See also
Practise it
Practise stemming until it is automatic: questions that explain every wrong option.
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