Arnold Steinberg, a polling expert, offers a good reason to think the latest poll showing Donald Trump leading the Republican presidential field is problematic. Here's an excerpt:

This survey’s sampling distribution by region, race and age is based on the 2010 census, which is dated and also not even relevant, because it hardly is indicative of overall turnout, certainly not of Republican turnout. If you are not defining the universe correctly, the survey is suspect. A sample correctly drawn from the wrong universe obviously does not represent reality. But let’s pretend the universe is correct and look at the sample. The sample is of 1000 “adults,” not voters, verified or not. At most, only 85 percent of those adults actually are registered to vote, and at most only 65 percent of those actual voters would vote in a primary. That means at most only 51 percent (.85 x. 65) of these one thousand adults, or any adult grouping within it, are relevant to the primary elections. The survey of “adults” relies on self-identification of Republicans, not actual voter registration. Worse, it adds independent “adults” who merely claim they would vote in a Republican primary, even if in some states with a closed primary they would be ineligible to vote. Regardless, the total claimed is 349 adults who are supposed Republican primary voters. Significantly, when we apply our conservative 51 percent assumption, that 349 becomes only 178 even possible Republican primary voters, with a margin of error of plus or minus about 7.5%. That’s sampling error, not errors arising from flaws. Remember, we don’t even know if the universe from which the sample is drawn is really appropriate. Regardless, that “statistical” theoretical margin of error doesn’t tell the full story. The interviewers tried to minimize bias by rotating the order of the 16 candidates. Thus, for the 178 respondents, we have only about 11 per rotation pattern. That’s hardly enough to allow for unknown reaction to each sequence of candidates. Finally, if all that were not enough, this survey was conducted from Thursday/ July 9 through Sunday, July 12. Friday, Saturday and Sunday generally are less reliable for polling and normally part of an extended study. And during these four specific days, Donald Trump dominated the news. For example, I heard him speak Saturday morning in Las Vegas at the Freedom Fest. He was entertaining and fun and well received. But in the hallways, the libertarians remained skeptical of his crony capitalism. But the media coverage of this event and his Arizona appearance was substantial.

Read the whole thing from Steinberg here.