Social scientists would love to work with indices that are perfectly reliable. Here's an example of a perfectly reliable "index." 

You went to your doctor. According to her very reliable (and expensive!) scale, you weigh 121 pounds. You go to a big-box store to buy an inexpensive scale for your bathroom. Alas, you cannot buy just one scale; you must buy 5. You take your 5 scales home and weigh yourself. All 5 scales give the exact same weight. 

Your friends Jill, Bob, Ellen, and Mike drop by. They try your 5 scales. Each scale gives the exact same weight for everyone. In the social sciences, think of the bathroom scales as opinions about an issue. Social scientists would not trust any single scale. They would take the average of all 5 scales as the most reliable indicator of your true weight (as well as the weight of your friends).

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