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The formula for percentage change in _any_ metric, without exception, is: (new_value - old_value) / old_value Some people flip the sign of their result, presumably because they associate "positive" with "good" or want to render a graph where "upward" bars are "good". Making this mistake consistently leads to tremendous confusion. This is particularly horrible when people confuse time and speed, which are inverse metrics. Flipping the sign in order to present the inverse metric is commonly done, but is mathematically incorrect and is often extremely misleading. That's why it's so important to use a standard (mathematically correct) convention for reporting a change in percentage of any metric. So, if I triple the size of the code, I'll now see +200%, not -200% which is mathematically impossible and shear nonsense.