“…The main motivations of expressing H 0 as in ( 7) are that (i) R pro t (β, u; θ * ) is based on unconditional moment restrictions, implying that we can avoid the use of tuning parameters such as bandwidths when estimating R pro t (β, u; θ * ); and (ii) R pro t (β, u; θ * ) depends on covariates only through the one-dimensional projection β ⊤ X, greatly reducing the dimensionality of the problem. Indeed, this dimension-reduction device has been proven valuable in many contexts that need to deal with a large number of covariates; see, e.g., Escanciano (2006), García-Portugués et al (2014), Sun et al (2017), Zhu et al (2017), and Kim et al (2020); for an overview, see Guo and Zhu (2017). However, it is worth mentioning that (7) involves not only a single process R pro t (β, u; θ * ) as is commonly the case in the specification testing literature (see Escanciano, 2008 for an exception), but J different processes R pro t (β, u; θ * ) associated with the treatment levels t. From (7), one natural way to proceed is to compute the generalized residuals marked empirical process based on the projections 1…”