Attribution determines how credit for a conversion is assigned to the touchpoints in a customer's journey. The model you select directly influences how Google Ads automated bidding allocates budget across campaigns, ad groups, and keywords. Choosing the wrong model can systematically over-invest in bottom-funnel tactics while starving the awareness and consideration campaigns that generate future demand.
The Problem with Last-Click
Last-click attribution assigns 100% of conversion credit to the final ad click before conversion. It is simple and deterministic, but it systematically undervalues every touchpoint except the last one. A customer might discover your brand through a YouTube ad, research your product through a generic search ad, and convert through a branded search ad. Under last-click, the branded search campaign gets all the credit, making it appear that YouTube and generic search contribute nothing.
Google deprecated most rules-based attribution models (first-click, linear, time-decay, position-based) in Google Ads as of September 2023, leaving two options: last-click and data-driven attribution.
Data-Driven Attribution (DDA)
Data-driven attribution is now the default and recommended model in Google Ads. It uses machine learning to analyze all converting and non-converting paths, then assigns fractional credit to each touchpoint based on its actual contribution to conversions.
How DDA Works
Google's DDA model evaluates the sequence and combination of ad interactions that lead to conversions versus those that do not. It considers factors including:
- Ad interaction type: Clicks, engaged views, and impressions are weighted differently.
- Time between interactions: Touchpoints closer to the conversion may receive more credit, but this is learned from data, not assumed.
- Number of interactions: The model accounts for path length and diminishing marginal returns.
- Device type: Cross-device journeys are stitched together using signed-in Google account data.
- Ad creative and format: Different ad formats may contribute differently at various stages.
DDA requires a minimum data threshold to function. Google Ads needs at least 300 conversions and 3,000 ad interactions in the past 30 days for a given conversion action. Below this threshold, the model falls back to a simplified cross-channel approach that requires less data.
Benefits of DDA
- Provides a more accurate picture of campaign contribution across the full funnel.
- Enables Smart Bidding strategies (Target ROAS, Maximize Conversions) to allocate budget more effectively.
- Reduces the systematic over-investment in branded search that last-click causes.
- Accounts for cross-device and cross-channel journeys.
Multi-Touch Attribution Beyond Google Ads
Google Ads DDA only attributes credit across Google Ads touchpoints. It does not incorporate non-Google channels (organic search, direct, social media, email). For a complete view of marketing contribution, enterprises need a cross-channel attribution solution.
Cross-Channel Options
- Google Analytics 4: Offers data-driven attribution across all traffic sources. However, it operates on its own model and may produce different results than Google Ads DDA for the same campaigns.
- Marketing Mix Modeling (MMM): A statistical approach that uses aggregate data (spend, impressions, conversions) to estimate channel contribution. Does not depend on user-level tracking and is privacy-resilient.
- Incrementality Testing: Geo-based or user-based holdout experiments that measure the true incremental impact of a channel. The gold standard for proving causation, not just correlation.
Choosing the Right Model
For most advertisers running Google Ads with automated bidding, data-driven attribution is the correct choice. It provides the most accurate signal to Smart Bidding algorithms.
Use last-click only in specific scenarios:
- Your conversion action has very low volume (under 300 per month) and you want deterministic reporting.
- You are running purely bottom-funnel direct-response campaigns with no awareness or consideration layer.
- You require exact, non-modeled attribution for financial reporting or regulatory compliance.
For enterprise advertisers, the recommended approach is to use DDA within Google Ads for bid optimization while supplementing with incrementality testing and MMM for cross-channel budget allocation decisions.
Sources
- Google Ads Attribution Models: https://support.google.com/google-ads/answer/6259715
- About Data-Driven Attribution: https://support.google.com/google-ads/answer/6394265
- Google Ads Deprecation of Rules-Based Models: https://support.google.com/google-ads/answer/13448534
- Google Analytics 4 Attribution: https://support.google.com/analytics/answer/10596866