The text is giving you a simple “map” for how to read a 2×2 table (a grid with 2 rows and 2 columns). It’s telling you what each box in the grid means, using letters. The 2×2 table idea Imagine you’re studying a group of people to see whether a certain factor (like exposure to smoke, a chemical, or a vaccine) is connected to a result (like getting a disease). The two things you compare are: - Exposure: exposed vs. unexposed - Disease: diseased vs. not diseased So the table has 4 boxes, and the note assigns a letter to each one. What each letter means − a = (exposed & diseased) People who were exposed and got the disease. - b = (exposed & not diseased) People who were exposed but did not get the disease. - c = (unexposed & diseased) People who were not exposed but still got the disease. - d = (unexposed & not diseased) People who were not exposed and did not get the disease. Quick example (to make it concrete) Suppose you’re studying whether living near a factory is linked to asthma: - a could be the number of people living near the factory who have asthma. - b could be those same people who don’t have asthma. - c could be people not living near the factory who have asthma. - d could be people not living near the factory who don’t have asthma. Why this note matters Whenever the text later refers to cells a, b, c, or d, this note tells you exactly what each one refers to—so you don’t get confused. If you want, paste the part of the document that uses the table, and I can help you interpret the results using this a/b/c/d setup.