EL PASO, Texas -

Scholars, lawmakers and people trying to decipher the proposed immigration bill are concerned the pathway to citizenship for millions of people living in the U.S. illegally is contingent on an "imaginary number."

The bill lays out a 13-year plan to citizenship for the estimated 11-million people living in the country illegally. Before that plan to legalization is set in motion, the bill stipulates the border must to be secure and it's the definition of secure that has raised questions by critics of the bill.

The proposed legislation calls for a path to legalization only be implemented until border security is 90% effective. The 90% effectiveness rate is defined by the bill as the number of apprehensions and turn backs in a specific sector divided by the total number of illegal entries.

"It makes all of that depend on this imaginary number at the border that we're really not sure how to calculate," said Josiah Heyman, Ph.D, a Professor and Chair of Anthropology at the University of Texas at El Paso. Heyman researches and publishes on Border Enforcement Policy.

His concern, shared by some lawmakers, is that there's no hard number on illegal entries because immigrants cross the border undetected, which means the key criteria to enforce the immigration bill is based on a guess.

The government currently measure illegal entries using apprehension numbers, aerial surveillance, turn backs and even tracks in the desert to generate an estimate, said Heyman.

Hayman also said the equation's lack of objectivity lends itself to partisan manipulation. "It's a politically charged number. It's making the legalization of people who are already here anyway, contingent on that number - that means that it ties everything to the politics of this mysterious number." Critics of the bill believe politicians can allege the border is safer if there's more or fewer apprehensions - depending on their political agenda.

Those who believe the 800-page bill does not address other border security measures, believe lawmakers are missing the biggest risks.

"The real security issues at the border are guns, drugs and money. That's associated with dangerous criminal organizations. We are completely ignoring the real risk to innocent civilians and we're focusing on mass migrants who want to chop vegetables in a restaurant."

Immigration to the U.S. is drastically down in the past decade. In fiscal year 2000, border agents apprehended more than 115,000 illegal immigrants in the El Paso sector, compared to fiscal year 2012, when they apprehended fewer than 10,000.