Before revision:  Thus, since the differentiation of the two causes has implications for nonparametric (point) identification and estimation of the conditional quantile function(s), we suggest a second test to identify whether the cause for rejection at the first stage was solely due to selection or not.
After revision:  Thus, since the differentiation of the two causes has implications for nonparametric (point) identification and estimation of the conditional quantile function(s), we suggest a second test to provide supporting evidence whether the cause for rejection at the first stage was solely due to selection or not.