What is Substitution bias?

Answer 1

The bias that arises when the index number formula used for an output PPI systematically understates average price increases because it does not take into account that producers seeking to maximize revenue from a given technology and inputs may shift production to items with above average relative price increases.

Or, the bias that arises when the index number formula used for an input PPI systematically overstate average price increases because it does not take into account that producers seeking to minimize costs with a given technology and output may shift purchases of inputs to items with below average relative price increases.

Reference: http://stats.oecd.org/glossary/detail.asp?ID=5698