Only applies to items that exist in both sets of data
Change in Price
( P₂ - P₁ ) Q₂
Change in Quantity
( Q₂ - Q₁ ) P₁
To further break out change in quantity
Change in Quantity - Volume Related
Q₂ ( Q₁ / Σ ( Q₁ ) ) - Q₁
Change in Quantity - Mix Related
Q₂ - Q₂ ( Q₁ / Σ ( Q₁ ) )