A 42-year-old woman from rural Maharashtra presents with a 6-month history of progressive dyspnea and cough. Chest X-ray shows bilateral upper lobe fibrosis. She reports exposure to silica dust while working in a stone quarry for 15 years. A case–control study is conducted to investigate the association between silica exposure and silicosis in quarry workers. Among 200 exposed workers with silicosis, 180 had documented silica exposure history. Among 200 unexposed controls without silicosis, 30 had documented silica exposure history. Which measure of association is most appropriate to calculate from this case–control study design, and what does it estimate?