Multi-numerology multi-carrier (MN-MC) techniques are considered as essential enablers for RAN slicing in fifth-generation (5G) communication systems and beyond. However, utilization of mixed numerologies breaks the orthogonality principle defined for single-numerology orthogonal frequency division multiplexing (SN-OFDM) systems with a unified subcarrier spacing. This leads to interference between different numerologies, i.e., inter-numerology interference (INI). This paper develops metrics to quantify the level of the INI using a continuous-time approach. The derived analytical expressions of INI in terms of mean square error (MSE) and error vector magnitude (EVM) directly reveal the main contributing factors to INI, which can not be shown explicitly in a matrix form INI based on discrete-time calculations. Moreover, the study of power offset between different numerologies shows a significant impact on INI, especially for high order modulation schemes. The finding in this paper provides analytical guidance in designing multi-numerology (MN) systems, for instance, developing resource allocation schemes and interference mitigation techniques.