Idaho Central Credit Union was the best-performing credit union in the U.S. for the fourth consecutive year.

S&P Global Market Intelligence ranked the nation's credit unions using five core financial performance metrics: member growth, net charge-offs as a percentage of average loans, efficiency ratio, asset quality and market growth. To be eligible for the ranking, a credit union had to report more than $500 million in total assets and a net worth ratio of at least 7.0% as of Dec. 31, 2015. Based on these criteria, 488 credit unions qualified for the ranking.

S&P Global Market Intelligence calculated each credit union's standard deviation from the mean for each of the ranking metrics. The standard deviations from the mean for each metric for each institution were then equally weighted and added together to calculate a performance score for each credit union. To help normalize the data and mitigate the impact of outliers, caps and floors were applied for each metric.

Idaho Central improved in multiple metrics in 2015. The Chubbuck, Idaho-based credit union's ratio of net charge-offs to average loans dipped 5 basis points to 0.09%, while its efficiency ratio dropped 9.17 percentage points to 61.45%. Idaho Central saw market growth — defined as growth in member shares and nonmember deposits — of 21.67%, compared to 19.15% in 2014. The credit union's assets grew by 30.50% to $2.42 billion in 2015, making it the 75th-largest credit union by assets as of Dec. 31, 2015.

Lake Michigan Credit Union took the No. 2 spot in the 2015 ranking, after coming in fifth for 2013 and 2014 and sixth for 2012. During 2015, the Grand Rapids, Mich.-based credit union decreased its net charge-offs-to-average loans ratio slightly and cut its efficiency ratio by 203 basis points to 63.04%. The credit union saw increases of 157 basis points in its member growth rate and 506 basis points in its market growth rate in 2015.

Third-place University of Iowa Community Credit Union improved from its No. 16 spot for 2014 and its No. 20 spot for 2013. Although the North Liberty, Iowa-based credit union's efficiency ratio increased by 78 basis points year over year to 48.17%, favorable changes in the other four metrics boosted the company's ranking.

The runners-up in the 2014 credit union rankings — Sugar Land, Texas-based Schlumberger Employees Credit Union; Jacksonville, Fla.-based First Florida Credit Union; and Green Bay, Wis.-based Capital Credit Union — did not make it into the 2015 top 50 list due to factors including significant declines in member and market growth rates.

The nation's largest credit union by assets, Vienna, Va.-based Navy Federal Credit Union, climbed 10 spots from 2014 to No. 138, while the second-largest credit union by assets, Raleigh, N.C.-based State Employees' Credit Union, fell 150 spots from the 2014 ranking to No. 323 in 2015.

Santa Rosa, Calif.-based Redwood Credit Union showed the most improvement from 2014's top 50 list, jumping 34 spots to No. 8. Mountain View, Calif.-based First Technology Federal Credit Union and Tukwila, Wash.-based Boeing Employees Credit Union each climbed 24 spots to Nos. 6 and 16, respectively.

Among the companies that featured in the top 50 lists for both 2014 and 2015, Wausau, Wis.-based Connexus Credit Union fell the most, dropping 27 spots to No. 33, followed by Albuquerque, N.M.-based Sandia Area Federal Credit Union, which fell 23 spots to No. 34, and Upper Marlboro, Md.-based NASA Federal Credit Union which slid 22 spots to No. 50.

Out of the 72 credit unions based in California that qualified for this analysis, 12 made it onto the top 50 list. Six of the 20 eligible credit unions in Washington were among the top 50.