Say I estimate the following probit model:
$$ins = \Phi(\alpha + \beta_1 age + \beta_2 educ + \beta_3 hg + \beta_4chronic + \beta_5 hisp + \beta_6 lin) + u$$
$ins = 1$ for any individual who has private health insurance, $0$ otherwise.
age = age in years.
educ = years of schooling.
$hg=1$ if health status self-assessed as good, 0 otherwise.
chronic = number of chronic conditions an individual has.
hisp$=1$ if Hispanic, 0 otherwise.
lin = natural log of household income.
I was if I were to perform a Wald test that for an individual with otherwise median characteristics, the marginal effect of age is unaffected by the number of chronic diseases the individual has, how do I derive the null hypothesis? (P.S., I can conduct the wald test quite easily in STATA, but my question is how do I state the null hypothesis?).