Figure 5. The item responses of K6 on log-normal scales.

(A) National RDD sample, (B) national RDD sample of twin pairs, (C) siblings of individuals from the RDD sample, and (D) oversamples from five metropolitan areas in the US. Using a log-normal scale, all four subsamples showed that the item responses for the 6 items followed a linear pattern between “a little of the time” and “all of the time.” In addition, the gradients of the linear patterns for the six items were similar to each other. To be specific, the lines for “a little of the time” to “some of the time” were relatively parallel to each other, whereas the lines for “most of the time” to “all of the time” were less so.