The
key assumption for
identification underlying all
of our estimates is the conditional independence assumption (CIA), which assumes that there are no
variables omitted from our matching process that affect both the choice
of textbook and the outcome (student achievement).
The link between tree ring properties and climate properties are very complex with multiple
variables and interactions among
variables, which makes the
identification of the temperature signal very difficult to statistically separate — especially when there are not clear measures
of the
key variables of interest including precipitation, temperature, soil conditions, soil conditions, diseases, shading, etc..