Definition
Email cannibalisation refers to the situation where email marketing captures conversions or engagement that would have occurred through another marketing channel in the absence of the email. If a subscriber would have purchased through a paid search ad or an organic visit but instead purchased through an email click, the email has cannibalised that channel's attributed conversion. Understanding cannibalisation is essential for accurate channel measurement and budget allocation.
Cannibalisation is most pronounced between email and channels that target the same audience with similar messaging. Email and paid social often serve overlapping audiences, especially when using email lists for lookalike audience targeting on Facebook or LinkedIn. When a subscriber receives both an email offer and sees a paid social ad for the same product, the channel that receives the click attribution may not reflect its true contribution. Similarly, email and organic search compete when subscribers search for a brand or product and find it organically but the email prompted the search in the first place.
Incrementality testing is the gold standard for measuring email's true impact. The most rigorous methodology involves suppression group testing, where a randomly selected holdout group of subscribers is excluded from email communications. The holdout group's conversion rate is compared to the treatment group's rate. Any conversion difference represents email's incremental impact. If the treatment group converts at 5% and the holdout group converts at 4%, the incremental lift is 1 percentage point, meaning 80% of the email-attributed conversions were incremental and 20% would have happened anyway.
Best Practices
- Run suppression group tests at least quarterly to measure incrementality: Select a 5-10% holdout group of subscribers and suppress all marketing emails to them for a defined period. Compare the holdout group's conversion behaviour to the active group. Regular testing captures incrementality changes as market conditions, audience behaviour, and cross-channel dynamics evolve.
- Use incremental metrics in channel performance reporting alongside attributed metrics: Report both attributed revenue and incremental revenue for email. The gap between the two numbers is the cannibalisation rate. Presenting both metrics gives stakeholders a complete picture and prevents over-investment in email based on inflated attribution numbers.
- Analyse cannibalisation by subscriber segment, not just overall: High-value, frequent purchasers typically show higher cannibalisation rates because they would be more likely to purchase through another channel anyway. Low-engagement, infrequent purchasers show lower cannibalisation and higher incrementality. Segment-level analysis identifies which subscriber groups email is genuinely influencing versus which groups would convert without it.
- Coordinate email and paid social campaigns to reduce overlap: When running email and paid social campaigns for the same offer, exclude email subscribers from paid social targeting for the offer's duration. This reduces cannibalisation between the two channels and provides cleaner attribution data for each channel's performance.
- Adjust budget allocation based on incremental rather than attributed performance: When email shows a high attributed ROI but a low incremental ROI, the channel appears more valuable than it truly is. Shift budget toward channels and campaigns with higher incremental ROI, even if their attributed ROI is lower, to maximise overall marketing effectiveness.
Related Glossary Terms
Email Analytics
The measurement, collection, analysis, and reporting of email performance data to understand subscriber behaviour and optimise campaign results.
Email Conversion Lift
Email conversion lift measures the incremental increase in conversion rate attributable to email campaigns using holdout group testing and statistical significance analysis.
Email Holdout Group
An email holdout group is a randomly selected subset of subscribers excluded from email campaigns to provide a baseline for measuring incremental lift in opens, clicks, conversions, and revenue.
Frequently Asked Questions
Email cannibalises organic search when an email prompts a subscriber to search for a brand or product and then click an organic result rather than the email link. Use tracking parameters on all email links and analyse the conversion path. If a subscriber opens an email, does not click, then searches for the brand and converts through organic search within 24-48 hours, that conversion may be email-influenced but attributed to organic.
Cannibalisation rates vary widely by industry, audience, and campaign type. E-commerce brands typically see 10-30% of email-attributed conversions being cannibalised from other channels. High-frequency buyers and loyal customers show 30-50% cannibalisation rates, while new or lapsed subscribers show 5-15%. These figures depend heavily on attribution model and measurement methodology.
A suppression group, also called a holdout group, is a randomly selected subset of subscribers who are excluded from all marketing emails for a test period. Their conversion behaviour represents the baseline that would occur without email. The difference in conversion rate between the active group and the suppressed group is the incremental impact of email. Any conversions attributed to email that also occur in the suppressed group represent cannibalisation.
Cannibalisation typically increases over time as subscribers become more familiar with a brand and develop purchase habits. A new subscriber who has only received three emails is more likely to be incrementally influenced by email than a five-year subscriber who has purchased 20 times. This is why incrementality testing should be conducted regularly and why suppression groups should be refreshed to maintain random representation.
Yes, cannibalisation is bidirectional. Other channels can capture conversions that email would have driven. For example, if a subscriber sees a paid social ad and then opens a marketing email and purchases, the conversion might be attributed to email but was actually driven by social. This is one of the challenges of single-touch attribution and a reason why multi-touch models and incrementality testing provide a more accurate picture of channel contribution.