Your ROAS looks great — but is it actually driving growth?
The Dangerous Seduction of High ROAS Every digital marketer has experienced the rush of checking a dashboard and seeing a Return on Ad Spend (ROAS) that looks like a statistical anomaly. A 10x, 15x, or even 20x return suggests that for every dollar you put into the machine, twenty dollars in revenue are pouring out the other side. In many boardrooms, this is the cue to open the champagne and double the budget. However, for the sophisticated growth marketer, a high ROAS is not always a cause for celebration. Sometimes, it is a warning sign. The fundamental question isn’t just “What is the return?” but rather, “Would this revenue have existed without the ad?” The gap between reported performance and actual incremental growth is where millions of marketing dollars are lost every year. When an ecommerce company hires a PPC agency, the honeymoon period usually consists of high conversion volumes and a healthy ROAS. On the surface, the strategy is a resounding success. But if you look closer, you might find that the campaign is simply standing in front of a door that was already open. If those conversions would have occurred anyway via direct visits or organic search, the paid campaigns are merely taxing the business rather than growing it. The eBay Experiment: A Lesson in Causal Lift To understand the limitations of ROAS, we must look at one of the most famous case studies in the history of paid search: the eBay experiment. In 2013, researchers from the University of California, Berkeley, teamed up with eBay to analyze the effectiveness of the company’s massive spend on branded search terms. At the time, eBay was spending millions of dollars bidding on its own name. Their internal metrics showed a massive ROAS. However, the researchers conducted a controlled experiment: they turned off paid search ads for the keyword “eBay” in specific geographic regions while keeping them active in others. The results were startling. In the regions where the ads were turned off, organic traffic picked up nearly 100% of the lost clicks. The revenue remained almost identical. The conclusion was clear: eBay was paying for traffic it already owned. Despite this evidence, many brands continue to spend heavily on brand keywords. Sometimes this is a defensive move to prevent competitors from poaching the top spot, but often it is a “safe” way to inflate reported ROAS. Platforms love these campaigns because they provide high-confidence conversions, but from a business growth perspective, they represent zero incremental value. The Black-Box Trap: Performance Max and Advantage+ As digital advertising moves toward total automation, the difficulty of measuring true growth has intensified. Modern advertising tools, such as Google’s Performance Max (P-Max) and Meta’s Advantage+, are essentially black boxes. They use machine learning to find the users most likely to convert, but they don’t necessarily prioritize finding *new* customers. Algorithms are designed to achieve the goal you set for them. If you tell an algorithm to maximize ROAS, it will find the path of least resistance to a conversion. Often, this path leads straight to your existing customers. Automation thrives on “safe” signals, which often results in the following: Brand Search Cannibalization: Algorithms bid aggressively on your brand name because those users are the most likely to buy. Aggressive Retargeting: The system serves ads to users who have already added items to their cart and were seconds away from checking out. Reporting Bias: Platforms claim credit for any user who saw an ad and eventually purchased, even if the ad had no influence on their decision. Without a way to measure incrementality, automation simply amplifies these non-incremental signals. You may see your ROAS climb, but your total business revenue remains stagnant. You aren’t scaling your business; you are scaling your platform spend. Incrementality: Measuring Causal Impact Incrementality is the gold standard for measuring marketing effectiveness. It refers to the “causal lift” created by a specific campaign. In simpler terms, it answers the question: “What changed because this campaign existed?” While platform attribution tells you which channel was the last touchpoint before a sale, incrementality tells you if the sale would have happened in a world where that channel was turned off. This is a much more useful lens for budget allocation. A channel can have a fantastic in-platform ROAS and still generate a weak incremental impact if it is merely harvesting demand rather than creating it. Think of it this way: Attribution is like a scoreboard in a basketball game. It tells you who took the last shot. Incrementality is like an advanced scouting report. It tells you how much better the team performs when a specific player is on the court versus when they are on the bench. If the team scores the same amount of points regardless of whether that player is playing, that player’s “incrementality” is zero, regardless of how many shots they take. The Difference Between Demand Generation and Demand Capture To master incrementality, you must distinguish between campaigns that create new demand and those that capture existing demand. High-funnel activities, such as YouTube awareness ads or social media prospecting, often have lower reported ROAS because they are introducing people to the brand for the first time. However, their incrementality is often very high because they are moving people who would never have considered your brand into the sales funnel. Conversely, bottom-of-funnel activities like branded search and retargeting often have astronomical ROAS but low incrementality. They are simply capturing the demand that was created by your high-funnel activities, your brand reputation, or word-of-mouth. The Hidden Metric: Marginal ROAS Even if you prove that a channel is incremental, you still need to know how much to spend on it. This is where Marginal ROAS comes into play. Marginal ROAS measures the return on the *next* dollar of spend, rather than the average return across the entire budget. Every marketing channel is subject to the law of diminishing returns. The first $1,000 you spend usually targets your “low-hanging fruit”—your most loyal customers