Chunk #13 — 2. METHODS — 2.3 Statistical Analyses

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Genetic etiology of the common liability to drug dependence: evidence of common and specific mechanisms for DSM-IV dependence symptoms.

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Descriptive statistics were obtained using SAS® (2002) and R© (2009). Differences in mean symptom levels between males and females were determined using weighted t-tests and Chi-square test in which the weights were based on the size of the family such that families with a set of twins was weighted 0.5 and families with an incomplete pair of twins was weighted as 1.0. Prior to biometrical modeling, symptom counts were regressed on age, age-squared, and age-cubed within each gender and the residuals scores were standardized within each gender. The logarithm of the scores (log(x+2)) was taken to better approximate a normal distribution. The newly created log-transformed age-adjusted scores were highly correlated (at least 0.97) with the standardized age corrected scores.