OBJECTIVES: Regarding compliance analysis numerous ratios can be found in international scientific literature with simpler or more complex methodology. In our analysis we tend to reveal, that choosing an adequate ratio is not sufficient itself, it is essential to know the difficulties and pitfalls of the data management and methodology to the objective assessment of the chosen ratio. The chief aim of our study to demonstrate factors in course of practical examples, which may substantially influence the results and the right conclusions, if these factors are modified. METHODS: The analysis is based on prescription refilling’s data of database of the Hungarian Health Fund in the field of the following indications: diabetes, COPD, oncology. From the ratios available in scientific literature, the PDC (Proportion of Days Covered) was chosen. The following aspects were considered as influencing factors: patient inclusion criteria (index date, time frame, criteria of refillings); DDD (WHO, SPC or real-world dosage to DOT); Gap (period without medication supply). A basic setting was established to calculate PDC ratio, then after changing each above specified parameters one by one (ceteris paribus), the ratio was recalculated. RESULTS: The PDC ratio shows huge variability recalculated by the different values of each parameters. Even more than 20% difference can be observed after modifying the gap (strict 1-day or permissive 30-day), or applying the SPC dosage instead of WHO DDD. In course of modifying the patient inclusion criteria both patient numbers and the ratio also show significant differences. CONCLUSIONS: Based on the results it may be concluded, that no general best practice can be observed, all settings have both advantages and limitations. It may be worth choosing the key parameters considering the specialties of each indications in order to draw conclusions as correct as possible with the focus of the original aim of the study.