I had a client once who paused all advertising for two months during a budget crunch. They expected sales to fall off a cliff. Instead, sales declined gradually, maybe 15% in month one and another 10% in month two. By month three, the decline accelerated sharply. What they were witnessing, without knowing the term, was the carryover effect.
The carryover effect is one of those concepts that should fundamentally change how you think about marketing budgets, but most marketers either don't know about it or treat it as an academic curiosity. I think that's a mistake, because understanding carryover is the difference between measuring advertising correctly and measuring it very wrong.
What Is the Carryover Effect?
The carryover effect (also called adstock or advertising decay) refers to the lingering impact of advertising on consumer behavior beyond the period in which the ad was delivered. When you run an ad campaign this month, some of its effect carries over into next month, and the month after that, diminishing over time but not disappearing instantly.
The concept was first formalized by Simon Broadbent in 1979, who introduced the term "adstock" to describe this phenomenon. His core insight was simple but powerful: advertising doesn't work like a light switch. It works more like a dimmer that fades slowly after you turn it off.
Monash University's marketing dictionary defines the carryover effect as "the rate at which the effectiveness of an advertising campaign diminishes with the passing of time." A carryover rate of 0.50 means that the impact of this month's advertising retains 50% of its effect next month.
Why Carryover Matters for Marketers
Here's why I think every marketer needs to understand this concept.
You're Probably Misattributing Your Results
If you're using last-touch attribution or even multi-touch attribution without accounting for time decay, you're crediting the wrong campaigns for your results. Some of your "organic" conversions this month are actually the carryover effect of paid campaigns from last month. Some of your "brand" lift is residual advertising awareness from campaigns that ended weeks ago.
Budget Decisions Are Wrong Without It
When you cut advertising spend and sales don't immediately drop, it's tempting to conclude that advertising wasn't working. But you might just be spending down your adstock. The real impact of the budget cut won't show up for weeks or months. Conversely, when you increase spend and don't see immediate results, you might be building adstock that will pay off in future periods.
It Explains Why Brand Advertising Has Different Economics
Brand-building campaigns have longer carryover effects than direct-response campaigns. This is a core finding from Les Binet and Peter Field's IPA research, and it explains why brand advertising often looks "inefficient" in short-term measurement frameworks but outperforms in the long run.
How the Carryover Effect Works: The Math
The standard adstock model uses a decay function. The simplest version is geometric decay:
Adstock(t) = Advertising(t) + λ × Adstock(t-1)
Where λ (lambda) is the decay rate, a value between 0 and 1. A higher λ means the effect persists longer.
Half-Life: The Key Metric
The most intuitive way to think about carryover is through the half-life: how long it takes for the advertising effect to decay to 50% of its peak.
Source | Typical Half-Life |
Academic studies | 7-12 weeks |
Industry practitioners (average) | 2-5 weeks |
Fast-moving consumer goods (FMCG) | ~2.5 weeks |
TV brand campaigns | 4-8 weeks |
Digital performance campaigns | 1-2 weeks |
Decay Rates by Channel
Marketing mix modeling practitioners typically use these rules of thumb for decay rates:
Channel Type | Typical Decay Rate (λ) | Carryover Behavior |
Search / Retargeting | 0.3 - 0.5 | Fast response, short memory |
Social media (paid) | 0.4 - 0.6 | Moderate carryover |
YouTube / Digital video | 0.5 - 0.7 | Longer brand impression |
TV advertising | 0.6 - 0.8 | Long memory-building effect |
Print / Out-of-home | 0.5 - 0.7 | Moderate to long, depends on frequency |
Email marketing | 0.2 - 0.4 | Very fast decay |
The pattern is clear: channels that build brand memory have longer carryover. Channels that drive immediate action have shorter carryover. This is why measuring advertising reach and frequency without considering time dynamics gives you an incomplete picture.
Carryover in Marketing Mix Modeling (MMM)
Marketing mix modeling has become the primary way companies account for carryover effects at scale, and the field is experiencing a significant renaissance. According to eMarketer, 46.9% of US marketers plan to invest more in MMM, making it the top measurement priority.
In MMM, the adstock transformation is applied to advertising spend data before regression analysis. This allows the model to capture how past advertising continues to influence current sales.
Modern Adstock Models (2025)
The original geometric decay model is giving way to more sophisticated approaches:
- Weibull distribution models — Increasingly preferred for long-duration campaigns, especially TV and brand advertising, because they allow for flexible peak timing (the effect can peak after the ad runs, not immediately)
- Meta's Robyn — Uses Weibull and Hill functions to model both carryover and saturation simultaneously
- Negative-binomial models — Allow more parameters to control the shape and scale of the decay curve
- Bayesian approaches — Google's Meridian and other Bayesian MMM tools incorporate prior knowledge about channel-specific decay rates
The Mechanisms Behind Carryover
Carryover isn't just one phenomenon. MBA Skool identifies several mechanisms that create carryover effects:
Memory formation. Advertising creates brand memories that persist in long-term memory and influence future decisions. This connects directly to brand awareness and mental availability.
Purchase reinforcement. Once someone has purchased a product (partly influenced by advertising), the purchase itself reinforces future purchasing behavior. The ad's effect cascades through the habit loop.
Distribution effects. Advertising can influence retailer stocking decisions, which continue to affect sales after the campaign ends. If your TV campaign convinces a retailer to give you more shelf space, that shelf space advantage persists.
Word of mouth. Advertising reaches direct viewers who then influence other potential buyers through conversation. The secondary influence extends the effective reach of the original campaign over time.
Real-World Examples
Super Bowl Advertising
Super Bowl ads are the most expensive single-exposure advertising buys in the world. Companies invest $7+ million for a 30-second spot, and they do it because the carryover effect is enormous. Research shows that brands which advertise during the Super Bowl see sustained sales lifts that extend weeks or months beyond game day, driven by the combination of massive initial reach, social media amplification, and earned media coverage.
Coca-Cola's 1980s Pause Test
Coca-Cola has conducted internal experiments where they pause advertising in specific markets. The consistent finding: sales don't drop immediately. They decline gradually over 4-8 weeks, then accelerate downward. This pattern matches the carryover model precisely and has been used to justify Coca-Cola's sustained brand investment strategy.
P&G During COVID-19
When many brands cut advertising during the 2020 pandemic, P&G maintained spend. They explicitly cited the carryover effect and long-term brand equity building as their rationale. By 2022, P&G had gained market share in multiple categories while competitors who cut spend were still trying to rebuild brand salience.
How to Use Carryover in Your Marketing Planning
I think there are three practical applications that every marketer should implement.
1. Adjust Your Attribution Window
If you're evaluating campaign performance on a 7-day or even 30-day window, you're probably undervaluing campaigns with strong carryover (typically brand and video campaigns). Extend your measurement window or use MMM to capture the full effect.
2. Budget for Consistency, Not Bursts
Because carryover means the effect of advertising accumulates over time, consistent spend patterns tend to outperform burst patterns for the same total budget. The exception is when you need to build sufficient frequency to break through (launch campaigns, seasonal pushes), where bursting can create a higher peak of adstock.
3. Use Channel-Specific Decay Rates for Budget Allocation
If TV has a decay rate of 0.7 and paid search has a decay rate of 0.3, then a dollar spent on TV generates more total impact over time than a dollar on search (all else being equal). This doesn't mean TV is always better; it means the time horizon of your business goals should influence your channel mix.
Carryover vs. Related Concepts
Concept | Relationship to Carryover |
Adstock | Synonym; the formal model that quantifies carryover |
Opposite direction; wearout is when repeated exposure reduces effectiveness | |
Frequency builds the initial effect; carryover determines how long it lasts | |
Reach determines how many people are affected; carryover determines how long each person is affected | |
Long-term carryover effects are one mechanism through which advertising builds brand equity |
Thought Leaders and Key Resources
- Simon Broadbent — Invented the adstock concept in 1979; his paper remains foundational
- Les Binet & Peter Field — Their IPA research on "The Long and The Short of It" proved that brand-building campaigns (high carryover) and activation campaigns (low carryover) require different time horizons for evaluation
- Meta Marketing Science team — Developed Robyn, an open-source MMM tool that implements advanced adstock modeling
- Google Meridian team — Google's Bayesian MMM framework incorporates prior knowledge about carryover rates
- Ashok Charan — Author of Marketing Analytics who has written extensively on dynamic effects in marketing mix modeling
FAQs
What is the carryover effect in advertising?
The carryover effect is the lingering impact of advertising on sales and consumer behavior after the advertising has stopped running. An ad campaign this month continues to influence purchase decisions next month, with the effect gradually decaying over time.
What is adstock and how does it relate to carryover?
Adstock is the formal model used to quantify the carryover effect. Coined by Simon Broadbent in 1979, adstock transforms advertising spend data to account for the fact that past advertising continues to influence current behavior. The terms are often used interchangeably.
How long do advertising effects last?
It depends on the channel and campaign type. Digital performance campaigns (search, retargeting) typically have half-lives of 1-2 weeks. Brand campaigns on TV can have half-lives of 4-8 weeks. Academic studies suggest an average range of 7-12 weeks for the total effect, while practitioners typically see 2-5 weeks for the half-life.
What is the half-life of advertising?
The half-life is the time it takes for the impact of advertising to decay to 50% of its peak level. A two-week half-life means that two weeks after a campaign ends, it retains 50% of its effect. After four weeks, 25%. After six weeks, 12.5%.
How does carryover affect marketing budget decisions?
Carryover means that cutting advertising doesn't produce immediate sales declines (creating false confidence) and increasing advertising doesn't produce immediate sales increases (creating false disappointment). Understanding carryover helps marketers set appropriate time horizons for evaluating campaigns and argues for consistent spend over time.
What is the difference between carryover and spillover effects?
Carryover refers to the time dimension: past advertising affecting future results. Spillover refers to the cross-channel dimension: advertising in one channel affecting results in another channel (e.g., TV advertising driving search volume). Both are important in marketing mix modeling.
How is carryover measured in marketing mix modeling?
In MMM, carryover is measured by applying an adstock transformation to advertising data before regression analysis. The decay rate parameter (λ) is estimated from the data, typically ranging from 0.3 to 0.8 depending on the channel. Modern tools like Meta's Robyn and Google's Meridian use advanced statistical methods (Weibull distributions, Bayesian priors) for more accurate estimation.
Does digital advertising have carryover effects?
Yes, but typically shorter than traditional media. Digital display and video ads have moderate carryover (decay rates of 0.4-0.7). Paid search and retargeting have shorter carryover (0.3-0.5). The key insight is that digital campaigns still influence behavior after they end, they just decay faster.
Sources & References
- Broadbent, Simon. "One moment in time," Admap, 1979.
- Monash University, "Carryover Effect in Advertising," monash.edu
- MBA Skool, "Carryover Effects," mbaskool.com
- DiGGrowth, "Understanding Carryover Effects In Marketing Mix Modeling," diggrowth.com
- Eliya, "Carryover Effects in Marketing: What They Are & How to Model Them," eliya.io
- Marketing IQ, "Adstock and Diminishing Returns," marketingiq.co.uk
- Impression Digital, "Adstock and the Long and Short of Advertising," impressiondigital.com
- Sellforte, "What are adstock and carryover effects?," support.sellforte.com
Written by Conan Pesci | April 4, 2026 | Markeview.com
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