A Log-Normal distribution function is the normal distribution for the logarithm of the variable. In linear scale it is a highly skewed distribution with a long tail in the high productivity side.

Why Does Chutes and Ladders Explain Hemoglobin Levels? Some Thoughts on the Normal Distribution Andrew J. Vickers, PhD Disclosures September 07, 2006 0 ...

Because the normal distribution is well understood and well documented, you can look up in a table how large the ratio must be to reach the 5 percent threshold (or any other threshold).