Significant Drop in Unpaid Channel Contributions After Upgrading to v1.1.1

[gitlingjin]

Hi Meridian team,

I wanted to flag an observation after upgrading from v1.0.7 (run in March 2025) to v1.1.1 (run in June 2025), using the same dataset and modeling configuration.

In the newer run under v1.1.1, we’re seeing:

• A substantial drop in unpaid channel contributions: two of our unpaid channels dropped from over 10% each to around 1% each.
• Some fluctuations in paid channel contributions, either upward or downward.

After reading through GitHub Discussion #790, I suspect the new default per-channel contribution priors for non-media treatments (introduced in v1.1.0) might be the cause of this shift.

Could you confirm if this change is expected behavior under the new defaults?
And is there a recommended way to replicate the previous (v1.0.7) behavior — e.g., by overriding or relaxing these new priors for unpaid channels?

Thanks so much in advance for your help!

[gousesb]

Hello @gitlingjin ,

Thank you for contacting us!

The shift in observations between v1.0.7 and v1.1.1, even with the same dataset and modeling configuration, is indeed expected behavior. This is due to the new default per-channel contribution priors for non-media treatments introduced in v1.1.0.

You can replicate the previous results by setting non_media_treatments_prior_type="coefficient" in your (ModelSpec). Alternatively, you could also consider using contribution priors and customizing the prior distribution of PriorDistribution.contribution_n.

Thank you

Google Meridian Support Team