A researcher wanted to investigate whether Mindfulness-Based Cognitive Therapy (MBCT) was an effective treatment for panic disorder. Fifteen patients diagnosed with panic disorder who had recently finished an 8-week MBCT course participated in the study. The researcher compared the self-reported frequency of panic attacks during the month prior to beginning MBCT with the frequency during the month immediately following the completed treatment.

Those patients who recorded fewer panic attacks in the month after treatment compared to the month before were categorised as 'improved' (+). Those who recorded more panic attacks were categorised as 'worse' (-). Patients who had exactly the same number of panic attacks in both periods were categorised as 'no change' (0).

Table 1: Patient outcomes following MBCT

Explain two factors that the researcher had to take into account when deciding to use the sign test. Refer to the investigation described above in your answer.