Why Polling on Marijuana Doesn't Tell the Whole Truth

Since March, six polls have been conducted on California's ballot proposition to legalize marijuana. The three that were automated showed the measure passing by double-digits. The three conducted by human operators showed it failing by a narrow margin. The results gap between the two methodologies widened when it came to minorities. Blacks and Hispanics heartily supported the measure when asked by robopolls but opposed it by 12 to 26 percent when asked by operators.

Nate Silver hypothesizes that automated surveys might have more trouble getting a representative sample of minorities -- or that respondents feel judged by a human on the other end of the line:

What if voters are more likely to admit their tolerance for marijuana to an automated script, which may create the feeling of greater anonymity? Marijuana usage remains fairly stigmatized in polite society in America, enough so that even liberal politicians like Barbara Boxer, Dianne Feinstein, Jerry Brown and Barack Obama have refused to state their support for legalizing the drug. But as most Americans between ages 20 and 55 have smoked marijuana, they may not consider it such a big deal in the privacy of their homes -- or the privacy of the ballot booth.

This might also explain why the split is larger among black and Hispanic voters. Marijuana usage is almost certainly more stigmatized when associated with minorities, and drug possession arrests occur much more frequently in minority communities. This is in spite of the fact that rates of marijuana consumption are only a smidgen higher among blacks than among whites, and are somewhat lower among Hispanics. (Although, note that the link I just pointed you to is also based on survey data, and so could be subject to some of the same biases.)

Read the full story at FiveThirtyEight.