This means that around $2.9 billion-worth of ad spend is under review, given that 90 percent of advertisers claim to be looking at their contracts.
Seventy percent of those surveyed use Agency Trading Desks – those within media agencies - as either the main way they buy programmatically or in certain markets, while 46 percent use Independent Trading Desks.
The WFA says that ITDs are growing in popularity, with usage going up 12 percentage points since it last conducted similar research in 2014.
Advertisers want greater transparency from suppliers in terms of how deals are done, especially in terms of how media agencies mark up online ad inventory they have bought directly from media owners. Sixty-two percent of those surveyed disagreed with the statement: "We have 'opted-in' to principal trading and are comfortable with the potential conflicts of interest."
Just over half (53 percent) of those surveyed said they have a "disclosed or transparent" relationship with their programmatic provider, while 33 percent said it was "non-disclosed or non-transparent."
"Programmatic has expanded rapidly and it's no surprise that the market and mechanisms that big brands use to spend through this channel are evolving," said the WFA's global media and digital marketing lead Matt Green in an emailed statement.
"The rise of in-house, hybrid models and independent trading desks demonstrates that the original trading model left much to be desired. The second generation of trading models is now being built and while agency trading desks still take the greatest share of digital spend there are now real alternatives being developed that give brands more control over data and technology alongside the wider push to ensure greater transparency," he added.