Pronoun reference

A pronoun must agree with its antecedent (ie the noun to which the pronoun refers) and the relationship between the two must be clear and unambiguous. Vague pronoun reference occurs when this relationship is not clear. For example:

Traditional extraction methods have many benefits, including producing oils with a low moisture content, but mechanical extraction methods produce higher yields. Chemical and nutritional analysis has shown that they vary widely depending on the variety of nut used.

There are several plural nouns in this paragraph, and it is not clear which noun the pronoun ‘they’ in the second sentence refers to. Using the noun 'yields' instead of the pronoun 'they' solves the problem in this case:

Traditional extraction methods have many benefits, including producing oils with a low moisture content, but mechanical extraction methods produce higher yields. Chemical and nutritional analysis has shown that yields vary widely depending on the variety of nut used.

Vague pronoun reference is a common cause of ambiguity in student writing. In some cases, replacing the pronoun with a noun is not enough to remove the ambiguity: if the meaning is still unclear after doing this, you may have to rephrase one or more sentences to achieve clarity.