Reliability means getting the same answer when measuring the same thing. Asking a single opinion question does not allow us to see how reliably that one opinion 'measures' a person's belief system about an issue like the death penalty. We have nothing to compare it to. 

One belief system holds that the death penalty is applied unfairly to the innocent, to persons of color, and to the poor. For example, the agree-disagree opinion in the illustration above states that 'many innocent people have been executed in the U.S.' The five people in the illustration above give different replies to this opinion. Some agree strongly (the man on the right) while others agree but not so strongly (the woman on the right).

Likewise, some disagree strongly (woman on the left) while others may disagree but not so strongly (man on the left). The woman in the middle is unsure. If we have several opinions designed to measure the same belief system (the death penalty is unfair, for example), then we would expect people to give consistent answers to those opinions. This consistency is what's meant by reliability.

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