I just took the survey. It was well-designed and easy. However, I think you need to define what you mean by low-cost airline. In the US, this term has a very specific legal meaning (a low-cost airline is an airline that either was created after, or did not provide interstate services until, the passage of the Deregulation Act). Up until the 90s there was a cost difference between legacy and low-cost airlines, but the legacies have gone through a few rounds of bankruptcy and shedded their higher costs, so at this point, the difference is little more than marketing.
Some of the airlines you have listed are considered ultra-low-cost airlines (Allegiant and Spirit) and are an entirely different category from JetBlue and Virgin, which are technically low-cost but in terms of price and service similar to the legacy carriers.