Comparison Period

A comparison period is a second time range you measure a metric against — this week versus last week, say — so a change gains meaning.

A single number is meaningless; '10,000 visits' is neither good nor bad until compared with something. A comparison period is the reference that turns a number into a judgment. But choosing the comparison period is itself analysis, not ceremony: comparing to the 'prior equal-length period' (the last seven days versus the seven before) tells the short-term trend, but is misleading where seasonality is strong; there, comparing to 'the same range last year' is more correct because it cancels the occasion and season effect. A common trap is comparing to an abnormal period: if last week was a holiday, this week's growth is fake. The comparison period must be equal in length and kind to the main period, or the percentage change deceives.