Fig. 2: Phenotypic (tetrachoric) correlation of item nonresponse among questions.

Each question has been recoded as dichotomous (1 = PNA or IDK, 0 otherwise). Each row and column represent the same question. We considered only questions that allow both the ‘Prefer not to answer’ and the ‘I don’t know’ options. Upper triangle, ‘I don’t know’; lower triangle, ‘Prefer not to answer’. The scale of possible correlations ranges from −1 to 1.