Validations errors are errors when users do not respond to required items.

A validation error occurs when you have Validation/Response Checking turned on for one of the questions and the respondent fails to answer the question correctly (numeric formatting , required response). The number represents the TOTAL number of times respondents have caused a validation error.

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Validation errors usually happen when you "require" a response to a particular question. If this number is particularly high and a lot of respondents are "starting" the survey but not "completing" it; this usually means that respondents get frustrated with the strict validation requirements.