Validation can catch a missing or malformed answer, but an overly strict rule can prevent a legitimate customer from contacting you. Define the problem the rule should catch before choosing it. A rule should protect the next step in the workflow, not merely make the data look uniform.

For example, a quantity field may need a sensible numeric range. A free-text fault description may need enough space for a useful explanation. A phone number should not be treated like a calculation just because it contains digits; international formats and leading zeros need thought.

Choose a rule and explain the correction

In a supported Google Forms question, open More and Response validation. Available rules depend on the question type. Short answers support options such as numbers, text, length and regular expressions; checkbox rules can limit the number of selections.

Write custom error text that tells the responder how to fix the answer. “Choose at most two options” is useful. “Invalid” leaves the person guessing. Keep the visible question instructions consistent with the actual rule.

For an email field, format checking can catch some typing errors. It does not establish that the mailbox exists or that the responder owns it. Do not treat a syntactically acceptable email address as an identity check.

Use three kinds of sample

Test a normal valid answer, a clearly invalid answer and an answer exactly at each boundary. If the maximum is 200 characters, check a sample at the limit and another just beyond it. If the field is optional, test leaving it empty as well.

Include realistic variations that the business accepts, such as names with punctuation or contact details copied with surrounding spaces. Use invented data for this rehearsal. Ask whether any rejected example actually should have been accepted.

Submit the completed test form and inspect what arrives in the response record. Recheck downstream handling when changing the rule. Validation that prevents one typo but breaks the enquiry process is not an improvement.

References: Google Forms response validation.