What must stay the same when calculating price elasticity of demand?

When calculating the price elasticity of demand, it is crucial to hold factors other than the price of the product being evaluated constant in order to isolate the relationship between price and quantity demanded. This includes holding consumers' income at a constant level and not letting it fluctuate, as changes in income can lead to changes in demand that are unrelated to price changes.

By doing so, the focus remains strictly on how the quantity demanded of the specific product changes in response to price changes while minimizing the influence of external factors. Keeping the price of the product not being evaluated constant ensures that the analysis pertains solely to the product in question, allowing for a clearer understanding of its demand sensitivity to price changes. This aspect of economic analysis is crucial for accurately determining the price elasticity of demand, as it reflects the responsiveness of consumers to price changes for that particular product in isolation from other variables.