Reach and Coverage of Custom Segments
One thing that is especially important to notice when adding custom segments to your campaigns is that reach will be reported as coverage. The reason for this is that the data regarding custom segments is based on unverified 3rd party sources, whereas regular demographic segments can be verified by cross-referencing data from national statistical banks. For instance, we know from Statistics Denmark that the 18-24-year-olds in Denmark represent around 9% of the online population, while the proportion of the online population that is in the market for buying a new car is an estimate based solely on survey answers.
To make a clear distinction between validation of the campaigns based on statistical banks data and the ones based on unverified 3rd party data, we distinguish between reach and coverage.
Also, when using the Custom Segments feature in AudienceReport, it is important to understand that data is a living thing - cookies get set, renewed and die all the time.
Hence, the custom segment needs to be up to date with our panel data. Otherwise, it would not be possible to report numbers for the segment. Thus, in order to add a custom segment to your campaign, the segment has to "exist" in the period in which the campaign is running.
This is generally guaranteed for all UserReport segments, but for 3rd party custom segments, it is the responsibility of the data provider to make sure that data has been uploaded simultaneously with campaigns running on those segments. Only this will ensure the validation of the custom segments of the campaigns.