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Response rate is completions over invitations, and inverting it sizes the invitation list. 620 responses from 5000 invitations is 12.4%, so reaching 1000 responses needs 8065 invitations - 3065 more than already sent. Response rates fall as a survey is re-sent, so the estimate is usually optimistic for later waves.
Response rate
rate = completed responses / invitations sent
Invitations needed
needed = target responses / response rate
Around 10-25% for cold email surveys, 25-50% for existing customers and over 60% for internal employee surveys with management backing.
Because the easiest respondents answer first. Marginal response rates on later reminders are typically half the initial rate.