Due to the rapid growth of applications that are based on Internet of Things (IoT) and real-time communications, mobile traffic growth is increasing exponentially. In highly populated areas, sudden concentration of numerous mobile user traffic can cause radio resource shortage, where traffic offloading is essential in preventing overload problems. Vertical handover (VHO) technology which supports seamless connectivity across heterogeneous wireless networks is a core technology of traffic offloading. In VHO, minimizing service interruption is a key design factor, since service interruption deteriorates service performance and degrades user experience (UX). Although 3GPP standard VHO procedures are designed to prevent service interruption, severe quality of service (QoS) degradation and severe interruption can occur in real network environments due to unintended disconnections with one's base station (BS) or access point (AP). In this article, the average minimum handover interruption time (HIT) (i.e., the guaranteed HIT influence) between LTE and Wi-Fi VHO is analyzed and measured based on 3GPP VHO access and decision procedures. In addition, the key parameters and procedures which affect HIT performance are analyzed, and a reference probability density function (PDF) for HIT prediction is derived from Kolmogorov-Smirnov test techniques.