Complex sentence: Legitimate surveys are usually unpaid (as with Gallup poll) or incentivized (as with YouGov); surveys where the respondent must pay or purchase products to join a panel are generally scams, as are sites that disappear before paying the participants.
Split into simpler sentences: Legitimate surveys are usually unpaid (as with Gallup poll) or incentivized (as with YouGov). Surveys where the respondent must pay or purchase products to join a panel are generally scams, as are sites that disappear before paying the participants.