What Are Friends for? A Longer Life

“People with stronger friendship networks feel like there is someone they can turn to,” said Karen A. Roberto, director of the center for gerontology at Virginia Tech. “Friendship is an undervalued resource. The consistent message of these studies is that friends make your life better.”

我的理解是：
一、样本是否具有代表性。那些更健康的人是否会更多的进入样本空间？会。

二、样本数据是否收集错误。

三、控制变量是否可控，非解释变量实在太多，很难控制，即使你使用compare-to-compare试验组合。