Representative of the population under study. In practice, "extra variables" are naturally present in every study. For example, surveying mall-goers in the mo

Representative of the population under study. In practice, "extra variables" are naturally present in every study. For example, surveying mall-goers in the morning vs evening. Or surveying people at one end of the mall vs the other. These variables are likely far more confounding than "must send a followup email". People asked in the evening might be more annoyed, but you shouldn't assume this a priori and decide to skip surveying them. Just as "busy people" in the studied population might forget to initially respond to an email and need a reminder. Just as a personal anecdote, I've dealt with this issue quite a lot. In my surveys, the people who respond initially are almost always eager to give glowing reviews. If we didn't send follow-ups, we would have extremely positively biased results. Maybe it is true that sending reminders makes respondents slightly more negative than they normally would be. However, we'd completely exclude "unsatisfied people" otherwise and the survey results would be worthless.