Lung Kuo, Shu, and Edward Ming-Yang Wu. “Highway Tunnel Air Quality Assessment Using Multivariate Statistical Classification on Factor, Cluster, and Discriminant Analysis”. International Journal of Engineering and Technology, vol. 7, no. 2.13, Apr. 2018, pp. 287-91, https://doi.org/10.14419/ijet.v7i2.13.12685.