Ratio Metric

A ratio metric divides one count by a base so that the result is independent of traffic size and comparable across periods and segments.

Choosing the denominator matters more than choosing the numerator: conversion per session and conversion per user are different numbers answering different questions, so the base belongs in the metric's name. A ratio removes size from the figure, and that strength is also its trap, since 100 percent on two sessions sits in the same column as 50 percent on ten thousand. The practical rule is to never report a ratio without its denominator and to read small segments cautiously. A third point is order of computation: take the ratio over period totals, not as the mean of daily ratios.