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This study investigates portfolio optimization within the Mean–Variance framework using alternative estimators for expected returns and risk. Based on daily data of VN30 stocks from March 2021 to December 2025, nine portfolio strategies are constructed by combining three return estimators (historical mean, CAPM, and Exponential Moving Average) with three risk estimators (sample covariance, Ledoit–Wolf shrinkage covariance, and semicovariance). Portfolios are optimized by maximizing the Sharpe ratio under realistic weight constraints and a post-optimization sparsification procedure to improve implementability. Empirical results indicate that portfolios using EMA-based expected returns combined with Ledoit–Wolf or semicovariance estimators deliver more stable and superior out-of-sample performance over 1-, 3-, and 6-month horizons. In contrast, strategies relying on historical mean returns with sample covariance suffer from severe performance degradation, while CAPM-based approaches underperform consistently. These findings underscore the importance of adaptive return estimation and robust risk modeling in emerging equity markets and offer timely implications for portfolio construction in Vietnam as it prepares for a potential upgrade to emerging market status.

Physical activity plays a crucial role in maintaining health and reducing stress, particularly among students in demanding disciplines such as nursing. This study examines how social media and self-efficacy influence exercise behavior and stress relief, drawing on the Social Cognitive Theory and Uses and Gratifications Theory. Data were collected from 295 Vietnamese undergraduate nursing students and analyzed using structural equation modeling (SEM). The results reveal that both social media use and self-efficacy significantly and positively affect stress relief, with exercise behavior acting as a mediating variable. The proposed model explains 69.2% of the variance in exercise behavior and 57.8% in stress relief. These findings suggest that exposure to fitness-related content and higher confidence in one’s ability to exercise contribute to greater engagement in physical activity and improved stress management. The study highlights the importance of integrating digital media and self-efficacy enhancement strategies into university health promotion programs. Universities should consider using social media platforms to share motivational content, facilitate online exercise communities, and organize student-led fitness initiatives. Policy-makers may also support campaigns promoting physical activity as an effective stress management strategy for health-related professions.

Grounded in Conservation of Resources theory, this study examines how psychological capital influences organizational citizenship behavior among Vietnamese IT expatriates in Taiwan, focusing on the mediating role of cross-cultural adjustment and the moderating influence of cultural intelligence. Survey data from 500 participants were analyzed using PROCESS Macro and AMOS. The results show that hope, self-efficacy, and optimism significantly predict cross-cultural adjustment, whereas resilience does not. All psychological capital dimensions positively predict organizational citizenship behavior, and cross-cultural adjustment partially mediates these relationships, indicating that psychological resources promote discretionary behaviors more effectively when expatriates achieve cultural ease. Cultural intelligence significantly strengthens the psychological capital, cross-cultural adjustment link, with the strongest effects observed for self-efficacy and optimism, highlighting the importance of cultural capability in resource deployment. This study advances expatriate research by integrating psychological capital, cross-cultural adjustment, cultural intelligence, and organizational citizenship behavior into a unified model within an East Asian technology context. Practical implications include developing psychological capital-cultural intelligence training, enhancing intercultural onboarding practices, and supporting migrant professional networks to improve expatriate retention and performance.