The word parts (affixes) in this test appear in more than one word in the most frequent 10,000 words in the British National Corpus.
Researchers believe that in order to use word part (affix) knowledge when guessing the meaning of unknown words, you need to at least: a) recognise a word part in the word, b) know the meaning of the word part, and c) know its function (i.e., part of speech)
This test measures each aspect of word part knowledge: whether you can recognise the forms, your knowledge of their meanings, and your knowledge of their functions.