Link to the research Group. Way down the page.
https://www.justfacts.com/immigration.asp
Highlights, The Mclaughlin Survey is specifically Hispanics, The Harvard/YouGov poll numbers are based on all illegal immigrants they do not separate Hispanics.
2013 McLaughlin Survey survey results and Census Bureau population estimates, 800,000 to 2.2 million non-citizen
Hispanics stated they were registered to vote in 2013
The Census Bureau counts only the number of non-citizens who respond to Census surveys, and some immigrants, especially unauthorized ones, avoid such surveys out of fear of exposing their immigration status.
Certain groups of immigrants often mispresent themselves as citizens in Census surveys
2008 Harvard/YouGov Survey 15% of non-citizens stated they were registered to vote these study results and Census Bureau population estimates, 2.8 million to 7.9 million non-citizens were registered to vote in 2008
The YouGov data was collected via an internet poll,
[1025] which are generally unreliable because they do not collect a random sample of respondents.
[1026] The Harvard study corrects for this by using a process called “matching.” Matching is a common procedure for turning non-random samples into random ones, but it relies on an “assumption” that there is “no difference” in how people would answer a survey if they have the same characteristics