The Public Policy Polling survey was done on February 2nd and 3rd of a random national sample of 517 likely Democratic voters, and has a margin of error of 4.3 points.
The Rasmussen Poll was done on February 3rd and 4th of a random national sample of 574 likely Democratic voters, and has a margin of error of 4.5 points.
The Quinnipiac University Poll was done between February 2nd and 4th of a random national sample of 484 registered Democrats, and has a margin of error of 4.5 points.
Bernie Sanders supporters have been blasting the results of the Quinnipiac poll over social media (and ignoring the other two polls). I can understand that -- it's a result they have been dying to see. But I would be careful about that. Quinnipiac could well be an outlier -- a flawed poll with hugely different results from all other polls. I wouldn't believe the Quinnipiac results until they are verified by a couple of other polls.