) : Measures the percentage of words outside a list of 3,000 familiar words. The 0.1579X0.1579 cap X
Based on your request, "1579x" appears in the context of a calculation used in readability assessment. Specifically, this is part of the Dale-Chall Formula for readability, which is used to analyze the difficulty of a text.
Essay: The Role of 1579x in the Dale-Chall Readability Formula