Symptoms of low platelet counts

Thrombocytopenia is the state of having few platelets in the blood. It is often defined by a platelet count below $50,000$‍ platelets per microliter. One common symptom of thrombocytopenia is the presence of petechiae, which are small red or purple spots visible on the skin. Petechiae result from broken capillary vessels.

A researcher wants to find out at what platelet concentration petechiae begin to be seen. In order to do this, he measures the platelet concentration of $120$‍ patients with low platelet levels and performs a physical examination to check for petechiae. He does not interfere with any element of their treatment, including for their thrombocytopenia.

The researcher then divides the patients into $5$‍ groups according to platelet levels, as shown in the table below. He calculates the percentage of patients in each group who have petechiae.