What's the most effective open auction network for incremental reach expansion on a DSP?

If you mean “which open auction supply source tends to deliver the best incremental reach at a DSP?”, there isn’t a single universal winner — it depends on geography, audience, and media quality filters. But in practice, the most effective open auction networks for incremental reach are usually the ones with the largest unique supply scale and strong identity/addressability coverage, such as:

• The Trade Desk OpenPath / open internet supply
• Google Authorized Buyers / AdX-style open auction inventory
• Amazon Publisher Services (APS)
• Large premium exchanges like Index Exchange, Magnite, PubMatic, and OpenX

If you want the shortest practical answer:

For incremental reach expansion, I’d usually start with:

1. Open exchange inventory from major SSPs like Index, Magnite, PubMatic, OpenX
2. Google open auction supply
3. APS if your audience overlaps well with commerce/retail-intent contexts

What tends to matter more than the “network”:

Incremental reach is usually driven by:

• Unique publisher coverage
• Low overlap with your current buys
• Identity match rates
• Frequency caps
• Audience extension quality
• Viewability / fraud controls
• Geo and device mix

Best approach

To find the best network for your DSP:

• Run a reach lift test by supply path
• Compare incremental unique reach vs. overlap
• Evaluate cost per incremental reached user
• Exclude already-saturated high-overlap supply
• Optimize toward new-to-file / new-to-campaign users

If you want, I can also give you:

1. a ranked list by DSP,
2. a framework for measuring incremental reach, or
3. a recommended open auction setup by objective.