Consider the following results of a multiple regression model of dollar price of unleaded gas (dependent variable) and a set of independent variables: price of crude oil, value of S&P500, price U.S. Dollars against Euros, personal disposal income (in million of dollars) :

Coefficient	t-statistics
Intercept	0.5871	68.90
Crude Oil	0.0651	32.89
S&P 500	-0.0020	18.09
Price of $	-0.0415	14.20
PDI	0.0001	17.32

R-Square = 97%

What is the interpretation of coefficient PDI?