My product has a funnel with steps A->B->C. Let's say I hold a constant of property P, but that property is not present on step A. How does Mixpanel behave here? What is my constant actually doing?
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If you hold a property constant in a funnel (A → B → C) but property P is not present on step A, Mixpanel's behavior depends on how property attribution and "hold property constant" are handled.
According to the documentation, when holding a property constant, "all the events must have the property, you can only select event properties that apply to all of the events in your funnel" — meaning Mixpanel will only allow you to select properties that are present on every event in your funnel steps. If property P is not present on step A, you should not be able to select it as a constant for the funnel. This is a guardrail to ensure the constant property logic works as intended: the user must perform all funnel steps with the same value for property P, and that value must be available on each step to enforce this rule Hold Property Constant.
If, for some reason, property P is inconsistently present (e.g., sometimes present on A, always on B and C), Mixpanel uses property attribution logic. By default, Mixpanel will "forwardfill" or "backfill" property values: if a property is present in earlier steps but not later, it is forwardfilled; if present in later steps but not earlier, it is backfilled. So, if you use "First Step Defined" attribution and property P is missing on A but present on B, Mixpanel will use the first non-null value it finds (from B) and apply it to the earlier step(s) for the purpose of breakdowns and filters Property Attribution.
However, for holding a property constant, the documentation is clear: you can only select properties that exist on all steps. If you attempt to hold constant a property that is not present on step A, Mixpanel will not allow you to configure the funnel this way.
Summary: Mixpanel requires that the property you want to hold constant is present on all funnel steps. If it is not, you cannot use it as a constant. If you try to use a property that is missing from a step, Mixpanel will prevent this configuration Hold Property Constant.
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If the property you want to hold constant (property P) is not set on all step A events, Mixpanel will only consider funnel entries where property P is defined on step A. In other words, users whose step A event does not have property P set will be excluded from the funnel when holding property P constant.
This is because, as per the documentation, "you can only select event properties that apply to all of the events in your funnel" when holding a property constant. The constant property logic requires that the property is present on every step for a user to be included in the funnel conversion calculation. If property P is missing on a user's step A event, that user's funnel attempt will not qualify for conversion under the "hold property constant" criteria Hold Property Constant.
So, in your scenario, only users whose step A event has property P set will be included in the funnel analysis when holding property P constant. All other users will be excluded from the funnel calculation.
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