“…To address such issues, some more flexible nonlinear modelling frameworks, and nonparametric and semiparametric estimation methodologies have been introduced in recent years by allowing the panel data "speak for themselves". (2006,2010), Wu and Zhang (2006), Li and Racine (2007), Cai and Li (2008), Henderson et al (2008), Mammen et al (2009), Li et al (2011a), Chen et al (2012aChen et al ( , 2012bChen et al ( , 2013aChen et al ( , 2013b, Su and Lu (2013).…”