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Suppletion is where the word-forms of the same lexeme have phonologically distinct stems. A study of thirty languages shows it to be surprisingly widespread, suggesting resistance to the pressure of paradigmatic levelling. While a major factor in its preservation appears to be the high frequency of the items that display it, two other factors are in operation, the type of inflectional category involved and the nature of the distribution of stems.
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Hippisley et al. (2004) studied this question.
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