Calculating the number of illegal immigrants is a tedious process. According to Jeffrey S. Passel, senior demographer at Pew Research Center, their methods have been polished over the years. However, the basic method for estimating illegal immigration statistics remained the same. Estimation is done by simply finding the difference between the total number of immigrants (provided by a census or national survey) and the number of lawful immigrants in the U.S. The difference is then the estimate for the unauthorized migrants.