Substitution bias arises because a fixed basket cannot capture buyers switching toward goods that have become:

1. A fixed basket assumes buying habits never change. 2. The price level is measured from a basket of goods people typically buy. 3. Two problems arise here: substitution bias and quality or new goods bias. 4. Buyers respond to price changes by shifting what they purchase. 5. A fixed basket misses that shift toward relatively cheaper goods. _Source: OpenStax Principles of Macroeconomics for AP Courses 2e (CC BY 4.0), Ch 8 'Inflation'_