Research studies in Human Robot Interaction (HRI) with social robots usually gather observational data in order to explore the dynamics of short and long-term interactions. The most common approach for the analysis of observational data is the proposal of a small number of behavioural units which frequency and/or duration is captured. As a consequence, comparing results between studies is difficult. The present manuscript proposes a procedure to assess the complete human-robot interactive activity. Experiences with two different robots were analysed using the novel instrumentation, leading to further considerations. Finally, general guidelines extracted from experimentation are proposed to assess the interaction quality between social robots and users. Further studies can be benefited from the proposed instruments, which are expected to be validated in different HRI contexts (e.g., school, hospitals, home) with different users (e.g., children, elderly, hospitalised people).