hart and risley's research has been criticised by scholars. paul nation criticises the methodology, noting that comparing the tokens (words produced) and number of types (number of different words) in unequal samples is not comparing vocabulary sizes. this is to say that the high socio-economic status samples naturally had a greater number of word types due to the greater number of tokens, because hart and risley extrapolated from tokens to an assumed greater number of types, but did not individually assess the number of types. the vocabulary sizes were cumulative, when it is possible that some new words produced were words that had been learnt in a previous month. in fact, hart and risley found no major differences in the "quality" of language produced between the different socio-economic groups.