After searching 400,000 nickels, I came up with the following attrition percentages:
99.74% for nickels from the 1920s
98.52% for nickels from the 1930s
96.31% for nickels from the 1940s
93.03% for nickels from the 1950s
75.45% for nickels from the 1960s
52.62% for nickels from the 1970s
and 48.13% for nickels from 1980-1981 period.
Overall for .999 nickels the average attrition rate is 63.72%.
When I started three years ago the average rate for my first 20 boxes was at 12.2% now it is at 8.1% for my last 20 boxes.
Cheers,