Avoid common calculation errors by learning when to calculate percentage decrease versus percentage difference.
Percentage decrease measures how much a value has dropped from its starting point. It requires a clear "before" and "after". The denominator is always the original value. Formula: ((Old Value - New Value) / Old Value) x 100.
Percentage difference is used when comparing two numbers without a chronolgical order or a clear baseline—for instance, comparing the heights of two mountains or the test scores of two students. Formula: (|Value A - Value B| / Average of A and B) x 100.
If you use a percentage decrease formula to compare two simultaneous things, your result changes depending on which item you randomly select as the "original". Returning two different percentages for the same comparison is confusing and mathematically unsound. Percentage difference solves this by using the unbiased average of both numbers.