All four major U.S. carriers ranked in the bottom half of a new study of "the consumer app experience" on 27 mobile networks in seven countries.

The report from Apteligent and the industry analyst firm STL Partners gauges mobile network speeds and reliability by measuring the performance of apps on the networks. The top five networks in terms of performance are in France and the U.K.; the worst networks are in Germany, Italy, Spain and the U.S.

T-Mobile had the best showing of any U.S. carrier but scored only 45 out of 100, 37 points fewer than France's Bouygues Telecom and four points below the median. Verizon placed second among American carriers with a score of 43, while AT&T scored 37 and Sprint finished in the bottom five carriers with a score of 33.

Each of the four U.S. operators was graded poorly on error rate, which the companies said was "a surprise given that many applications have been developed by U.S.-based startups and established companies, and the vast majority are hosted in the datacenters of U.S. Internet giants Amazon and Google."

The report is the inaugural effort from STL Partners and Apteligent, which dubs itself "a mobile app intelligence platform." The companies announced the launch of their mobile network experience index, which is branded MobiNEX.

STL is a London-based firm whose clients include Sprint and Deutsche Telekom, which owns T-Mobile and holds substantial shares in several European operators.

"A lot of people spend more time using apps than anything else on their phones, but how can they compare what they get today with what they really could be getting?" Chris Barraclough of STL Partners asked in press announcement. "The index shows in simple terms which operators are likely to do the best job for people."

Verizon's network is often cited as the best in the U.S., although its smaller rivals have closed the network gap as they build out and densify their LTE networks.

For more:

- see this press release

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This article was updated April 27 to mention some of STL's carrier clients.