Final answer:
The proportion of test takers who get an item correct calculates the item difficulty index, which can indicate the difficulty level of the question. Percentiles, such as the 70th, are also used to describe how a score compares with others.
Step-by-step explanation:
When you examine the proportion of test takers who get an item correct, you are calculating a statistical measure known as the item difficulty index in the context of test analysis. This measure is used to determine how many students answered a specific question correctly, and it can be used as an indicator of whether the item was too easy, too hard, or appropriately challenging. For instance, if 70% of students answered a question correctly, the item difficulty index would be 0.70, suggesting that the question was of moderate difficulty.
In the context of percentiles, if on a 20-question math test the 70th percentile for the number of correct answers was 16, it means that 70 percent of students answered 16 or fewer questions correctly, indicating that answering more than 16 questions correctly would place a test-taker above the 70th percentile, typically considered a good performance.