Final answer:
Stairs should not have more than a 3/8 inch difference in riser height for safety and uniformity.
Step-by-step explanation:
When talking about stairs/steps, there should be no more than 3/8 inch (9.5 mm) difference in the riser height to ensure uniformity and safety. Uniform riser heights create a rhythm that pedestrians can anticipate when ascending or descending staircases. In your example, you mentioned that the riser height is 15 cm. If there were differences between risers, it could pose a tripping hazard or discomfort for individuals using the stairs, especially in environments such as public buildings or when carrying heavy items.