There are three types of web analytics metrics:
Count: the most basic unit of measure; a single number, not a ratio…Often a whole number (Visits = 12,398), but not necessarily (Total Sales = $52,126.37.).
Ratio: typically, a count divided by a count, although a ratio can use either a count or a ratio in the numerator or denominator. (An example of a ratio fabricated from ratios is “Stickiness.”) Usually, it is not a whole number. Because it’s a ratio, “per” is typically in the name, such as “Page Views per Visit.” A ratio’s definition defines the ratio itself, as well as any underlying metrics.
KPI (Key Performance Indicator): while a KPI can be either a count or a ratio, it is frequently a ratio. While basic counts and ratios can be used by all Website types, a KPI is infused with business strategy – hence the term, “Key” — and therefore the set of appropriate KPIs typically differs between site and process types.
A fourth type of definition is included for terms that describe concepts instead of numbers.
Dimension: A general source of data that can be used to define various types of segments or counts and represents a fundamental dimension of visitor behavior or site dynamics. Some examples are event and referrer. They can be interpreted the same as counts above, but typically they must be further qualified or segmented to be of actual interest. Therefore these define a more general class of metrics and represent a dimension of data that can be associated with each individual visitor.
Metrics are measured across the dimensions. A metric can apply to three different universes:
- Aggregate: Total site traffic for a defined period of time.
- Segmented: A subset of the site traffic for a defined period of time, filtered in some way to gain greater analytical insight: e.g., by campaign (e-mail, banner, PPC, affiliate), by visitor type (new vs. returning, repeat buyers, high value), by referrer.
- Individual: Activity of a single Web visitor for a defined period of time.
Reprinted from Web Analytics Association’s white paper Web Analytics Definitions (PDF)