Background. Dependence on nicotine-containing products is of interest to regulatory agencies, as it bears on continued use of regulated products. However, dependence is a highly complex construct, requiring sound application of the science of behavioral measurement (psychometrics). While there are widely used scales available to measure adults' dependence on cigarette smoking, attempting to apply these scales to assessing dependence on ENDS, and applying them to new populations, such as youth, raises significant psychometric challenges. One such example is the use in the FDA and NIH sponsored Population Assessment of Tobacco and Health (PATH) youth survey of the Wisconsin Inventory of Smoking Dependence Motives (WISDM) scale. This scale was initially developed and validated to measure cigarette dependence in adults, but PATH used six adapted WISDM items to measure cigarette and ENDS dependence in youth. It is unknown whether use of these six items to assess and compare dependence on ENDS and cigarettes in youth is psychometrically appropriate. Objective. This analysis aimed to determine the reliability and validity of the scale scores obtained from the 6-item scale in assessing and comparing cigarette and ENDS dependence in youth who were either dual users of cigarettes and ENDS or exclusive users of either product. Where psychometrically appropriate, analyses compared dependence on cigarette smoking to dependence on ENDS use. Methods. A comprehensive psychometric evaluation of the 6-item scale was conducted, including assessing reliability, convergent/discriminant validity, item response function (IRT methods), internal structure (dimensionality), and scale invariance (CFA methods). Sample. Data were obtained from respondents to waves 3 and 4 of the PATH Youth survey who were currently (past 30 day) either dual users (n=106) or exclusive users of cigarettes (n=142) or of ENDS (n=367). Results. Initial analyses indicated that two items measuring perceived positive effects (i.e., "helps me feel better," "helps me think better") did not fit with the scale. With these items deleted, the scale was found to be unidimensional and reliable and valid for comparisons among youth dual users. The analyses showed that in this population, ENDS use was associated with lower dependence than was cigarette smoking. It was also determined, however, that the scale was not psychometrically appropriate for comparisons among youth exclusive users. Conclusion. The findings from this study reinforce the need to formally evaluate psychometric properties of scales used when assessing and comparing dependence across various populations or products. Without this, their use could lead to inaccurate assessment of dependence, and ultimately influence adoption of ill-advised public health policy.