It is not accurate that a Bear Stearns study found that there were 30 million illegal immigrants in the USA.
Instead the Bears Stearns study The Underground Labor Force
Is Rising To The Surface by Justich and Ng stated:
The number of illegal immigrants in the United States may be as high as 20
Undocumented immigrants are gaining a larger share of the job market, and
hold approximately 12 to 15 million jobs in the United States
In Coulter's book, she correctly reports the 20 million figure from Bear
Stearns and then states:
Combining Justich and Ng's conclusion that there were 20 million illegal aliens here in 2005 with the estimate of Pulitzer prize winners Barlett and Steele that another 3 million illegal immigrants would enter in 2006, plus at least another 3 million illegals coming in each year throughout the following decade - surely a low estimate - would mean there are at least 30 million illegal immigrants in the United States today.
So she is stating 3 million per year with no justification for 2007 to present, when this number could have actually been negative some of those years, particularly around 2009 (see DHS report below).
There are other sources that have estimated 30 million. The 2006 book Minutemen: The Battle to Secure America's Borders estimated 30 million and Californians for Population Stabilization estimated 20 to 38 million. However, the Minutemen book counts children of illegal immigrants who were born in the USA, which is clearly contrary to the definitions of "illegal" and "immigrant". Immigrants that were illegal but have recieved amnesty in previous amnesty events are also included.
The Californians group's report is actually a collection of 4 separate papers. For example 1 of the 4 is Illegal Aliens: Counting the Uncountable by James H. Walsh, former Associate General Counsel of the Immigration and Naturalization Service (INS) in the United States Department of Justice. This report estimates 38 million:
My estimate of 38 million illegal aliens residing in the United States is calculated, however, using a conservative annual rate of entry (allowing for deaths and returns to their homelands) of three illegal aliens entering the United States for each one apprehended. My estimate includes apprehensions at the Southern Border (by far, the majority), at the Northern Border, along the Pacific, Atlantic, and Gulf of Mexico coasts, and at seaports and airports. Taking the DHS average of 1.2 million apprehensions per year and multiplying it by 3 comes to 3.6 million illegal entries per year; then multiplying that number by 10 for the 1996–2005 period, my calculations come to 36 million illegal entries into the United States. Add to this the approximately 2 million visa overstays during the same period, and the total is 38 million illegal aliens currently in the United States.
However, this method is highly sensitive to selection of the number "3".
Another of the 4 papers, How Many Illegal Aliens Are in the U.S.? - An Alternative Methodology for Discovering the Numbers, doesn't really calculate an independent number but looks at how reasonable the 20 million figure reported by others is.
The Pew Research Center estimates 11.2 million in 2012 and 11.3 million in 2013. This is essentially the same as what the Department of Homeland Security (DHS) reports (11.5 million for 2011). These are the most widely reported values.
The DHS report at least clearly explains the methodology. The total number of foreign born persons in the USA is estimated from American Community Survey data. From this is subtracted DHS data for persons who immigrated legally or obtain legal status after immigrating illegally. Allowances are made for some undercounting of foreign born, mortality of legal/legalized immigrants and emigration. Also, everyone arrived before 1980 is assumed to be legal. So this "subtraction" technique is sensitive to under-response of foreign born persons to surveys. It is basically assuming 95% of foreign born are counted in the survey (90% of illegal immigrants and almost 100% of other immigrants). The DHS report specifically acknowledges that if only 80% of illegal immigrants responded to the survey the number could be 13.9 million. The Bear Stearns report says that from interviewing illegal immigrants they determined a low number would respond to such a survey, and that the DHS does not get the supposedly correct ~20 million value for this reason.