If the hunt for conversions has led you to consider behavioral retargeting, don't waste time working with more than one ad network.

Behavioral retargeting campaigns are often constrained by target population size, which leads advertisers to run retargeting campaigns across multiple ad networks in order to garner additional reach. But the fact is most networks buy inventory from the same or a similar pool of sites, which means there is a lot of overlap in audience between networks. A far more efficient use of budget is to run campaigns across one large, high-quality network. It's the best possible way to optimize reach while controlling frequency and cost.

Why retargeting?
User-based online behavioral retargeting identifies website visitation as an explicit demonstration of interest in an advertiser's products and services. Users that have visited the advertiser's site are thus targeted as they surf other sites with tailored messages intended to drive re-visitation and conversion.

More than two-thirds of behavioral ad targeting online today is retargeting-based. Its popularity is based on three factors: 

1. Behavioral retargeting data is readily available to the advertiser. Retargeting data is extracted from the marketer's own web traffic. So rather than acquiring third-party behaviors of interest, an advertiser simply provides its own behavioral data to an ad network.
2. Retargeting is easy to buy and affordable. Virtually every ad network offers retargeting campaigns. And because there are no data costs involved, retargeting is often available at a relatively low CPM.
3. Retargeting is effective. Retargeting campaigns are targeted to potential customers at the bottom of the sales funnel, when they are already researching and considering an advertiser's products. Advertising campaigns targeted at the top of the funnel to create awareness are rarely as effective without consistent re-messaging. An Advertising.com retargeting campaign promoting subscriptions for a newspaper achieved a conversion rate nearly 10 times that of a run-of-network campaign -- delivering nearly the same volume of subscriptions via 1/20th the total impressions of the run-of-network buy. For another Advertising.com campaign, ads were retargeted to consumers who had visited a microsite for a new car model. The advertiser sought to bring those interested consumers back to the site to configure a car and/or complete an information request. The result: retargeting produced 12 times the number of conversions and 23 times the number of information requests as demographic targeting.

Less is more.
Many retargeting campaigns are constrained by limited traffic to the advertiser's website, creating a relatively small targetable population for the campaign.

Marketers often try to overcome this challenge by running simultaneous retargeting campaigns on multiple networks. Unfortunately, most network audiences overlap. Many networks acquire inventory from similar sources and reach a similar audience. This redundancy means that essentially the same audience will be shown advertisements again and again, without significant additional reach. 

For example, Advertising.com reaches more than 90 percent of the uniques seen by many of the other popular advertising networks. If cross-visitation of networks is compared, the additional reach supplemented by multiple networks is shown to be minimal, particularly with the largest advertising networks.  (See Figure 1).

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A single large network can provide tremendous reach without creating frequency problems. The largest networks reach the vast majority of internet users, providing excellent probability that any particular user will be successfully, but not excessively, retargeted somewhere on the network.

Given the high incremental cost of each additional network and the low incremental reach it can add, it is hard to justify a multiple-network approach. In general, few new audience members are reached by expanding campaigns to new networks.

Bigger is better.
Consolidating retargeting campaigns on a single large advertising network offers the best balance of cost and reach. Retargeting campaigns can be highly effective, and reach is the key to maximizing their performance. However, running retargeting campaigns on multiple networks does not improve their efficacy. This strategy provides limited additional reach and prevents effective management of impression frequency.

Brent Halliburton is director of network strategy, Advertising.com. Read full bio.