Year

Correlation degree

r1

r2

r3

Quality level

2011

0.528

0.650

0.815

Ⅲ level

2012

0.433

0.502

0.347

Ⅲ level

2013

0.529

0.810

0.538

Ⅱ level

2014

0.137

0.623

0.090

Ⅱ level

2015

0.422

0.846

0.457

Ⅱ level

2016

0.467

0.816

0.439

Ⅱ level