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Source: Brian, via Wikimedia Commons.

Believe it or not, when you get an email from an airline asking you to "rate your experience from your recent flight." the airline is watching closely.

As The Wall Street Journal discussed last month, the response rates to email surveys for flights can get as high as 20%, much higher than the standard email survey goal of 5%. Apparently, when folks have positive or negative experiences on an airline, they like to have their voices heard.

Net promoter scores
Designed by Bain & Company consultant Fred Reichheld, Net Promoter Scores, or NPS, are one of the most closely tracked metrics of these email surveys. In essence, the NPS measures how likely a flier is to recommend an airline to someone else, on a scale of 1 to 10. 

According to the Journal, officials at Southwest (NYSE:LUV) and Virgin America monitor NPS results vigilantly. And executives from both JetBlue (NASDAQ:JBLU) and Delta (NYSE:DAL) "are convinced NPS results correlate to company profits, since stronger loyalty means stronger ticket sales." 

The email surveys have been responsible for a number of changes by the airlines -- from decisions about charging for checked baggage to the types of music made available on the airplane's radio. According to the JournalAlaska Airlines (NYSE:ALK) actually began pouring wine from full-size bottles instead of using single-serve bottles after hearing from customers. 

Notice any airlines left out?
It's interesting, though, that the article made no mention of airlines like Spirit (NASDAQ:SAVE) or Allegiant. Both airlines are classified as "ultra-low-cost" carriers.

In essence, that means that there is a base fee that is usually lower than the fees other carriers charge, but you could be charged for nearly any amenity you can think of -- from $100 fees for carry-on bags to $10 fees to print your boarding pass.

It should be no surprise that when we look at the rankings from airline review site Skytrax, Southwest, Virgin America, and JetBlue lead the way -- all with rankings above 7.5 out of 10. Allegiant comes in at a modest 5.5, and Spirit is rated at a super-low 2. 

An industry evolving
It's clear that the industry is becoming three-pronged. On one hand, airlines like Southwest and JetBlue are keeping costs low while doing all they can to provide transparency to their process and excellent service. That's why the surveys are so important to them.

On the other are the ultra-low-cost carriers, who -- in some instances -- get more than 40% of their revenues from "add-on" fees.

Somewhere in the middle are the legacy carriers.

However the industry evolves over the coming years, it's somewhat heartening to know that airlines are listening to customers, so think twice before deleting that customer review the next time it pops up in your inbox.

Fool contributor Brian Stoffel has no position in any stocks mentioned. The Motley Fool recommends Southwest Airlines. The Motley Fool owns shares of Spirit Airlines. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.