What is the average compressability of Earth's crust?

Some averages are useful, and some are not. This one would not be useful. Not to decide where to live, or construct a highway. But if I spend an average of$50 for groceries each week, I know how much money to budget for food. This could be useful.

The average lifespan for a male in the US in the years 2005 to 2010 was 75.6 years. An insurance company could use this number to predict how much they should charge for a life insurance premium.

The average human height differs from region to region. The tallest man to live on the planet was 8 ft 11 inches tall, while the shortest man stands at 1 ft 8 inches. The average human height in the United States is 5 feet 9.9 inches, for males, and 5 feet 4.3 inches. Only a very few people are of "average" height. So what could you do with the average numbers? Not to decide on the ceiling height in a room. Not to design a chair. Not to decide how high a light switch on the wall should be.

Although I cannot remember the specific numbers, rock compressibility is generally low. Porous rocks have higher compressibility while high-grade metamorphic rocks have almost no compressibility. Since the Earth's crust is approximately 70% mafic (e.g. the ocean basins) and 30% felsic (about a grano-diorite for the continents), compressibility of the Earth's crust is very low.