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Always ignore the sunk cost when making decisions
A sunk cost is not worth considering in decision-making processes because it is a cost that has already been incurred and cannot be recovered or changed. No matter what future actions you take, the sunk cost will remain the same. Therefore, it should not affect your future decisions because it is irrelevant to them.
For example, let's say you have purchased a non-refundable ticket to a concert. On the day of the event, you fall ill and are considering whether to go or not. The money spent on the ticket is a sunk cost. Whether you decide to go or not, the money cannot be recovered. Therefore, your decision should be based on your current health condition and enjoyment potential, not on the spent money.
Including sunk costs in decision-making can lead to irrational decisions, as you may feel compelled to commit to a choice because of the money or time you've already invested.