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Free flights are good. Free seats on the flights you want are better.

Because airlines have different rules about how customers earn and claim awards and sometimes limited seating availability on popular flights, airline mileage programs are notoriously difficult to value. Seat upgrades and free flights make these programs attractive — but what airline is most likely to have the seats you want available on the flights you need to take?

Ideaworks and CarTrawler answered this question, and the results may surprise you. Many airline reward programs have changed in how they value miles or points. Most once rewarded customers for the number of flights they took and the distance traveled. Now, many rewards are based on how much customers spend on their tickets. Out of 25 airlines included in the 2018 Reward Seat Availability survey, eleven programs are based on ticket prices.

It’s not bad news. Being rewarded more when you spend more is fairer. Also, competition from low-cost carriers has pushed traditional airlines to lower the points or miles you have to trade-in to book flights.

“For example, reward prices as low as 12,000 miles roundtrip were found on select Delta routes in the US, where the previous price was 25,000 miles roundtrip,” IdeaWorks and CarTrawler state.

The 2018 Overall Reward Availability rankings show that Southwest Airlines’ Rapid Rewards program is still the best. It’s the only airline to earn a perfect 100-percent score, as it did in last year’s rankings. Air Canada’s Aeroplan and Turkish Airlines’ Miles&Smiles rank second and third with 96.4 percent and 95 percent availability respectively.

The American Airlines AAdvantage program was most improved, rising 27.8 points over last year’s report to rank ninth, with 82.1 percent seating availability.

Here’s how other U.S. airlines performed:

Canada’s WestJet made the rankings for the first time this year. Its WestJet Rewards program ranked 20th with 57.1 percent availability. Low-cost long-haul airline Norwegian also made the IdeaWorks CarTrawler rankings for the first time. Its Norwegian Reward program was eighth on the list with an 84.3 percent chance that you’ll find seats to claim. Norwegian was beaten by the third newcomer in the rankings, China Eastern’s Eastern Miles program, which ranked seventh and scored 89.3 percent availability.

Maybe you’re saving your airline rewards for long-haul flights, helping to cut down costs on those vacations you’ve dreamed about. IdeaWorks and CarTrawler also ranked airlines by availability of long-haul flight rewards.

Turkish Airlines’ Miles&Smiles program topped that list with 98.6 percent availability, an improvement of 31.4 points over last year. Air Canada and Norwegian are tied in second place with 94.3 percent availability. The Scandinavian Airlines (SAS) EuroBonus program comes in at the bottom, with only 7.1 percent availability.

Since airlines are moving to rewards based on fares, Ideaworks and CarTrawler found a way to measure the overall value of rewards programs. The “Rewards Payback” metric measures how much value North American airlines give back for the money customers spend.

The best news is that Ideaworks and CarTrawler say airline rewards programs have improved over last year. Hopefully, that trend will continue.