You have the following dataset.

The formula in option B calculates the percent difference in sales relative to the same quarter the previous year. It subtracts the sales of the same quarter from the previous year from the current year’s sales, and then divides this by the previous year’s sales. This gives a percentage change, which aligns with what is shown in the Calculation field of your dataset. The LOOKUP function returns the value of a specified offset from the current row in the partition. In this case, the offset is -4, which means the value of the same measure four rows above the current row. This corresponds to the same quarter of the previous year, assuming the data is sorted by quarter. References:

LOOKUP Function - Tableau

Table Calculations - Tableau

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