Stemming and Lemmatizing

Sometimes when analysing natural language it is necessary to normalise the text. For example converting everything to lower case ensures that 'Inform' and 'inform' are not treated as different words. Sometimes it might also be necessary to treat 'inform' and 'informed' as the same. In those cases stemming and/or lemmatizing will be necessary. But these processes can give unexpected results.

In natural language programming a stemmer will remove affixes from a word leaving only the stem. NLTK comes with a number of in-built stemmers. Comparison of the standard stemmers:

word	Porter	Lancaster	Snowball
fasten	fasten	fast	fasten
classify	classifi	class	classifi
awaken	awaken	awak	awaken
differentiate	differenti	​differenty	differenti
duplicate	duplic	duply	duplic
specialise	specialis	spec	specialis
lying	lie	lying	lie

The Porter and Snowball stemmers generate the same result but Porter is for English only. The Lancaster stemmer produces very different results.
Lemmatization is simmilar to stemming except it always generates a valid word rather than a stem. For example:
classify gives classify rather than classifi
differentiate gives differentiate rather than  differenti
in the case of words like believes it gives belief whereas the Porter stemmer will give believ