Take, as an example, the question of nighttime sleep length. In this data, the average six-month-old baby sleeps ten hours a night. Great—that’s about what we saw in the studies I mentioned earlier. What about the baby at the 25th percentile (this would be a baby who didn’t sleep much)? Nine hours. What about the 75th percentile? Eleven hours. Now, what about the whole range of the data for six-month-olds? It turns out, in the data they see babies who sleep as few as six hours at night, and babies who sleep as many as fifteen hours. This makes things a bit clearer: at least part of the reason
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