The rationale and the measurement model for the SUD risk index (transmissible liability index, TLI) derivation procedure have been described in detail elsewhere (Vanyukov et al. 2003a (link), c (link)). Briefly, it is based on the known transmissibility of liability to SUD related to illicit drugs and on the application of item response theory (IRT). SUD liability is largely non-specific (common), both phenotypically (most of the variance is shared in common for various classes of illicit drugs) and genetically (twin data support the overwhelming contribution of nonspecific genetic variation) (Kendler et al. 2003a (link)). Inasmuch as SUD risk is transmissible (mostly due to its heritability), children’s characteristics that discriminate groups with affected and nonaffected parents are likely to be indicators of children’s transmissible SUD liability, which could be then analyzed by IRT. IRT (e.g., Embretson and Reise 2000 ) is a psychometric test theory that relates the performance of an examinee on a test item to a latent trait that the test is intended to measure. This relationship (e.g., in a simple case, between the trait and the probability of a correct response) is described by an item response function (IRF). While ability level is a characteristic of the examinee, performance also depends on parameters characterizing items themselves and defining the IRF. In the widely used two-parameter model, these are the location (difficulty) parameter (b, the trait value at which the probability of a correct response exceeds 0.5), and the discrimination parameter (a, proportional to the slope of the IRF at the point b on the trait scale). These parameters allow for taking into account that different items have different difficulty and different ability to discriminate between values of the trait. In contrast to the classical psychometric test theory, IRT provides testable models. A data-fitting IRT model provides estimates with features that are uniquely valuable for the trait measurement: item parameters are invariant of the sample (subpopulation) of the subjects (the trait distribution does not influence the estimates), and trait estimates are invariant of items used.
The TLI derivation method involves using a large set of items (303 in the current study) from numerous psychological and psychiatric instruments (here, 24), originally selected in CEDAR based on their potential for measuring variables related to SUD risk and psychopathology. These items were submitted to conceptual (identification of item groups judged to indicate core psychological traits), factor and item response theory (IRT) analysis to derive theoretically based unidimensional constructs (here, 19) characterizing individual behavior/personality (e.g., antisociality, attention, mood). The HAR and LAR groups were then compared on these constructs. This comparison relates the constructs to parental SUD liability and, inasmuch as liability is transmissible, to the child’s own SUD liability (see details in Vanyukov et al. 2003a (link)), consistent with the construct validity of TLI [its ability to capture the construct of transmissible liability; this and other standard research validities as defined, e.g., in Nunnally and Bernstein (1994) ]. The constructs demonstrating significant group differences were retained for further analysis. The items that are indicators of these constructs were then submitted to factor analysis to both ensure the presence of a single dominant dimension and further reduce the item set. Exploratory factor analysis of the set thus derived estimated a ratio of 3.2 of first to second eigenvalues, consistent with the unidimensionality of the TLI scale. Confirmatory factor analysis with weighted least squares method confirmed the unidimensional factor structure of the TLI scale, a prerequisite for IRT modeling. The 45-item set thus selected (see the listing inAppendix ) for sons of the probands at age 10–12 was used to assess the quality of items and estimate TLI in this study, using IRT.
Whereas this item set includes—by design—many items that have long been known to be related to SUD risk, the procedure has selected from disparate diagnostic and psychological instruments a large comprehensive initial list of potentially useful and the most relevant items, selected out many redundant ones, thus enabling substantial data reduction without a loss of information, as well as calibrated the items as indicators of the unidimensional transmissible liability trait. Because the liability distribution shifts to the right as the population matures from prepuberty, age was regressed out of the TLI in the twin sample.
The TLI derivation method involves using a large set of items (303 in the current study) from numerous psychological and psychiatric instruments (here, 24), originally selected in CEDAR based on their potential for measuring variables related to SUD risk and psychopathology. These items were submitted to conceptual (identification of item groups judged to indicate core psychological traits), factor and item response theory (IRT) analysis to derive theoretically based unidimensional constructs (here, 19) characterizing individual behavior/personality (e.g., antisociality, attention, mood). The HAR and LAR groups were then compared on these constructs. This comparison relates the constructs to parental SUD liability and, inasmuch as liability is transmissible, to the child’s own SUD liability (see details in Vanyukov et al. 2003a (link)), consistent with the construct validity of TLI [its ability to capture the construct of transmissible liability; this and other standard research validities as defined, e.g., in Nunnally and Bernstein (1994) ]. The constructs demonstrating significant group differences were retained for further analysis. The items that are indicators of these constructs were then submitted to factor analysis to both ensure the presence of a single dominant dimension and further reduce the item set. Exploratory factor analysis of the set thus derived estimated a ratio of 3.2 of first to second eigenvalues, consistent with the unidimensionality of the TLI scale. Confirmatory factor analysis with weighted least squares method confirmed the unidimensional factor structure of the TLI scale, a prerequisite for IRT modeling. The 45-item set thus selected (see the listing in
Whereas this item set includes—by design—many items that have long been known to be related to SUD risk, the procedure has selected from disparate diagnostic and psychological instruments a large comprehensive initial list of potentially useful and the most relevant items, selected out many redundant ones, thus enabling substantial data reduction without a loss of information, as well as calibrated the items as indicators of the unidimensional transmissible liability trait. Because the liability distribution shifts to the right as the population matures from prepuberty, age was regressed out of the TLI in the twin sample.
