The AdSense Host API reporting system exposes the flexibility and customization capabilities of the AdSense website. Reports are generated by specifying a list of dimensions and metrics.
Dimensions
Each time AdSense displays an ad, every event associated with that ad is logged with a set of attributes: date, country, channel, ad unit, etc. These attributes are called dimensions.
Dimensions are used for grouping purposes in reports.
Metrics
When a report is requested, the logged events are added together according to the chosen dimensions. Metrics represent the different measurements you can make on that data.
Some metrics are calculated, instead of being measured directly.
Using multiple dimensions wisely
The generated report consists of a series of rows, each having one or more fields. Adding a new metric to a report produces a new column but adding a dimension multiplies the number of rows.
For example, a report with DATE as a dimension will generate a set of rows, one row for each day with activity. Adding a new dimension, like CUSTOM_CHANNEL_NAME, will generate a row per day and per existing channel. As a guideline it's fine to use as many metrics as you like, but try not to use more than two dimensions per report.
Incompatible dimensions and metrics
There are some metrics and dimensions that cannot be combined in a single report. For example, a conflict will arise when trying to fetch URL and custom channels at the same time. This limitation is the same as in the AdSense website (Performance reports tab).