/* Heteroskedasticty Test 
   The example data contains:
   pcon: Petroleum Cosumption for state i
   price : Price per galloin in state i
   reg : Motor vehicle registrations in the state in thousands
   pop : Total populations in thousands 
*/

use http://bigblue.depaul.edu/jlee141/econdata/econ375/hetero_gas

regr pcon price reg
predict ei, resid
scatter ei reg

hettest, iid /* the Breusch-Pegan Test */
estat imtest, preserve white  /* the White Test */

regr pcon price reg, robust /* Use heteroskedasticty correct s.e. */

/* Change variables to per capita */
gen pc_pcon = pcon/pop
gen pc_reg  = reg/pop

regr pc_pcon price pc_reg 
hettest, iid /* the Breusch-Pegan Test */
estat imtest, preserve white  /* the White Test */