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None of these effects in itself predicts what will happen to actual quantity of goods demanded ( the number of units purchased ) as prices change — they refer only to preferences or propensities to purchase.

The actual effect on quantity demanded will depend on the range of other goods available, their prices, and their substitutabilities for the goods concerned.

The effects are anomalies within demand theory because the theory normally assumes that preferences are independent of price or the number of units being sold.

They are therefore collectively referred to as interaction effects.