December 15, 2025

Introducing Multi Touch Attribution in Source: First Touch, Position Based, and Time Decay Models

Source Adds 4 New Attribution Models

Up until now Source used First Touch Attribution to calculate attribution.  This means when a person visits your site via social media, and then again via search to convert, then all of the credit as attributed to Social Media (the first touch).

This is the best model for many use cases as your user learned about your company via Social, then used search to come back to convert. 

The social campaign is what influenced the customer, and search was just a means to an end.  You should be spending more dollars on your Social campaign.

But what if your marketing strategy follows The Rule of 7

In this case, each touch is considered influential to the customer's ultimate conversion.  So a Multi-Touch attribution model is appropriate.

How does it work?

On the surface, it's easy. Apply a fractional weight to each conversion based on the attribution model.

In practice, there were two fun challenges while building this - determining the session (the touch), and handling aggregate data from Ad APIs.

Challenge 1: Sessions

Say someone visits your site via paid channel, finds your blog interesting, and clicks through a few articles.  That entire session will be attributed to the paid channel as one touch.  If they close their browser and come back for a few more articles, that’s another touch.

Sessions are intuitive to talk about, but there are no browser APIs available to capture, so we have to use heuristics.

Source uses your browser’s Session storage and HTTP referrer headers to determine if a click is a new touch or part of an existing session. New sessions get recorded with a Session ID and are what Source uses to calculate a touch.

Here's the decision flow:

Challenge 2: Crunching the Numbers

First and last touch attribution means the first or last touch receives 100% of the conversion credit. In the above example, the Referral touch gets a weight multiplier of 1, and everything else gets 0.
Multi Touch attribution uses different algorithms to weight the interactions. Source provides a Position Based option, which gives 80% weight to the first and last touches, then spreads the remaining 20% among the middle touches. And 2 Time Decay options which give the highest weight to the most recent touch.

You can select either Exponential Time Decay, which is in line with how Google reports, or you can choose Linear Time Decay which is more gradual.

Attribution Data

There are two types of attribution metrics Source handles - pixel metrics, and external metrics

Pixel Metrics

Pixel metrics come from the Source pixel you installed on your site, so the information is complete and Source can calculate exact numbers.  For pixel metrics, Source calculates weights directly against event data.  Two touches with 0.5 weight will have an overall conversion influence of 1.

External Metrics

Source pulls metrics from APIs via your advertising channels - Google Ads, Meta Insights, etc.  These data sources come in aggregate, without specific attribution data, and Source needs to apply statistics and confidence levels to paint an accurate picture. Using pixel data, Source knows the overall weight paid and organic channels had during your report time frame.  That weight is applied to the aggregate values returned from Ad source APIs.

Extra!

Statistics is also how Source is able to provide organic search results at a user level.  Google Search Console provides high level time and geographic information about organic queries.  Source uses that information to make an educated estimation about which query is associated with a given contact.