if I do attributed by UTM Source and a user’s first event utm source in the window was not set, and then they come again with a utm source, how is that mp going to attribute it (first touch)?
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If you use "Attributed by UTM Source" with a first-touch attribution model in Mixpanel, the platform will attribute the conversion to the first event within the attribution lookback window that contains a value for the UTM Source property.
If the user's first event in the window has UTM Source as "(not set)" (i.e., the property is missing or undefined), and a subsequent event within the same window has a valid UTM Source, Mixpanel will attribute the conversion to "(not set)"—not to the later event with a valid UTM Source. This is because first-touch attribution always credits the first qualifying event in the window, regardless of whether the property is set or not. The "(not set)" value indicates that the user arrived without a UTM parameter, which is interpreted as organic or direct traffic.
This behavior is confirmed in the documentation:
You may see a “(not set)” segment in your report when using the Attribution feature. This occurs when the attribution property is missing from all events being evaluated for the user.
Computed Properties: Attribution FAQ
So, in your scenario, the conversion will be attributed to "(not set)" (organic/direct), not to the later event with a UTM Source value. If you want attribution to only occur when a UTM Source is present, you would need to filter out "(not set)" segments in your analysis.
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You are correct—according to Mixpanel documentation, when using first-touch attribution with a property like UTM Source, Mixpanel will attribute the conversion to the first event in the lookback window where the UTM Source property is defined (i.e., not null or "(not set)"). If the first event in the window does not have a UTM Source, Mixpanel will look for the next event where UTM Source is set and use that value for attribution.
This is described in the documentation for "First Step Defined Attribution and Forwardfilling":
Select First Step Defined to attribute the first property value to the whole funnel, regardless of whether the property value changes in subsequent steps. First Step Defined is not the same as the first step of the funnel, rather it is the first time the property is given a non-null defined value.
For example... With “First Step Defined”, the first color will be applied to the entire funnel, in this case, “red” seen on View Item.
Funnels Advanced Concepts: Property Attribution
So, if the first event has no UTM Source and the second event does, the conversion will be attributed to the UTM Source value from the second event.
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Thank you for clarifying that you are referring to the "Attributed by" functionality in Insights reports.
Based on the available knowledge sources, the "Attributed by" breakdown in Mixpanel allows you to attribute credit for a conversion to touchpoints along the user journey, using properties like UTM Source. The attribution is calculated based on the first step of the funnel or the first event in the lookback window where the property is defined—not simply the first event in the window.
Specifically, for first-touch attribution in Insights reports: - Mixpanel will attribute the conversion to the first event within the lookback window where the UTM Source property is set (i.e., not null or undefined). - If the first event in the window does not have a UTM Source, Mixpanel will look for the next event where UTM Source is present and use that value for attribution.
This is consistent with the "First Step Defined" logic described for funnels, which also applies to property attribution in general:
First Step Defined is not the same as the first step of the funnel, rather it is the first time the property is given a non-null defined value Funnels Advanced Concepts: Property Attribution.
Therefore, in your scenario: If the first event in the window does not have a UTM Source and the second event does, the conversion will be attributed to the UTM Source from the second event.
If you need more technical details specific to Insights (not Funnels), the knowledge sources do not provide further explicit clarification, but the described behavior is consistent across Mixpanel's attribution features Computed Properties: Attribution.
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