Embedded in our smiley face survey is a tracking pixel. When it's downloaded by your recipient's email client, we record that he or she has "seen" our survey. When the pixel isn't downloaded, we are only able to infer that the survey has been seen if the recipient clicks one of the smiley face images to provide feedback.
Response rate, is therefore, an approximation of how often your recipients see our smiley face survey, expressed as a percentage, and is calculated like this:
# of Reviews Received / # of Surveys Seen = Response Rate %
You can also read more about how to increase your response rate.