Satisfaction of psychological needs has been linked to human flourishing and improved wellbeing and basic psychological needs are central to self-determination theory (SDT) and include autonomy, relatedness, and competence. Accurate distinction between stable and dynamic aspects of these psychological needs is necessary for the development and assessment of interventions aiming to maximize satisfaction of these needs. The widely used Basic Psychological Needs Satisfaction scale (BPNS) was developed to measure the degree to which people feel satisfaction of the three basic psychological needs but its ability to reflect stable and dynamic aspects of needs and generalisability of assessment scores of were not thoroughly examined by implementing an appropriate methodology. Generalisability theory (G-theory) has been recommended as an appropriate statistical method to evaluate the state-trait distinction while providing reliability analysis and accurate evaluation of sources of measure error. G-theory was implemented to distinguish between state and trait aspects of psychological needs and to assess the reliability of the BPNS. A longitudinal person by item by occasion observational design was applied to an adequate sample of 116 participants who completed the 21- item BPNS at three time points with a one-month interval between assessments. The total BPNS showed acceptable reliability and generalisability of scores in assessing the need satisfaction trait across sample population and occasions (G = 0.75-0.88) while individual subscales of the BPNS appeared less reliable due to dynamic nature of needs reflected by these subscales. A brief subscale to measure the most dynamic needs as a state was developed using items reflecting dynamic aspects of psychological need. The overall good reliability of the total BPNS and dynamic properties of the individual subscales, suggest the overarching latent trait of neediness that varies across individuals while specific needs are state like and therefore can shift from one aspect to another depending on circumstances of individual’s life.