Pew Research Center designed this study to assess the current state of online survey sampling methods – both probability-based and opt-in – and determine their accuracy on general population estimates ...

Any scientist who studies groups of people knows that the characteristics of the "sample" -- the group of people selected for the study -- can profoundly impact the study's findings. To produce the ...

EXPLAINS. WE’VE COLLECTED BLOOD SAMPLES, URINE SAMPLES, TOENAIL SAMPLES, AND WE’VE COLLECTED MANY OF THESE SAMPLES REPEATEDLY OVER TIME. THE NUMBER CATALOGED HERE IS IN THE MILLIONS SINCE 1976, MORE ...