SPECIAL SECTION
Subtypes Versus Severity Differences in Attention-Deficit/Hyperactivity Disorder in the Northern Finnish Birth Cohort

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ABSTRACT

Objective

To investigate whether behaviors of inattention, hyperactivity, and impulsivity among adolescents in Northern Finland reflect qualitatively distinct subtypes of ADHD, variants along a single continuum of severity, or of severity differences within subtypes.

Method

Latent class models, exploratory factor models, and factor mixture models were applied to questionnaire data of ADHD behaviors obtained from the Northern Finland Birth Cohort (NFBC). Latent class models correspond to qualitatively distinct subtypes, factor analysis corresponds to severity differences, and factor mixture analysis allows for both subtypes and severity differences within subtypes.

Results

A comparison of the different models shows that models that distinguish between a low scoring majority class (unaffecteds) and a high scoring minority class (affecteds), and allow for two factors (inattentive, hyperactive-impulsive) with severity differences provide the best fit.

Conclusions

The analysis provides support that a high-scoring minority group (8.8% of males and 6.8% of females) likely reflects an ADHD group in the Northern Finland Birth Cohort, whereas the majority of the population falls into a low-scoring group of unaffecteds. Distinct factors composed of items of inattention and hyperactivity-impulsivity are evident for both sexes with considerable variability in severity within each class.

Section snippets

Subjects

The sample used in the current analysis consists of 6,622 16- to 18-year-olds drawn from the larger NFBC (Järvelin et al., 1993) who completed the 2001-2003 assessment including the postal screening for ADHD and approved of its use in the present study.

Procedure

The Strengths and Weaknesses of ADHD-Symptoms and Normal-Behavior scale (SWAN; Swanson et al., 2001b) questionnaire was sent to the families as part of a larger NFBC survey conducted in 2001-2003. Parents were asked to complete the form and

RESULTS

Goodness-of-fit measures shown in Table 1 (for females) and Table 2 (for males) illustrate the results of fitting the different models. Lower values of AIC and BIC indicate better fit. General results regarding the model comparison and the choice of the preferred model are similar for the two sexes. Because previous analyses have not directly compared LCA and FA models, we organize the discussion of each table by first comparing models within a given model type (e.g., LCA, FA, FMM) to show what

DISCUSSION

The statistical approach used in the present study shows that FMMs provide the best fit to the behavioral measures of inattention, hyperactivity, and impulsivity derived from a parent questionnaire obtained on adolescents in the NFBC. These best fitting models differentiate between the unaffected majority of the cohort and the potentially affected minority, and, in addition, allow for systematic variation in severity. LCA is based on the assumption that individual differences in the observed

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    This research was supported byNational Institute of Mental HealthgrantsMH063706(Smalley, Jarvelin) andMH01966(McGough), and by the Yrjö Johnsson Foundation (Hurtig), the Juselius Foundation, and the Academy of Finland.

    Disclosure: Dr. Moilanen is a member of the Eli Lilly Strattera Advisory Board, Finland. Dr. McGough receives grant research support from Eli Lilly, McNeil, New River Pharmaceuticals, Novartis, Shire, and Pfizer and is also a consultant to Eli Lilly, Novartis, and Shire and serves on the speakers' bureaus of Eli Lilly, McNeil, Novartis, and Shire. Dr. Swanson has received research support and honoraria from, served on the speakers' bureaus of, and served as a consultant or advisor to Alza, McNeil, Janssen, UCB, Cephalon, Eli Lilly, Novartis, and Shire U.S. The other authors have no financial relationships to disclose.

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