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Global errors are errors that affect overall sentence organization (for example, wrong word order). They are likely to have a marked effect on comprehension (R. Ellis, 2008, p. 964).
Local errors are errors that affect single elements in a sentence (for example, errors in the use of inflections or grammatical functors [sic] (R. Ellis, 2008, p. 970).
Source: Ellis., R. (2008). The Study of Second Language Acquisition. 2nd ed. Oxford: OUP.