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Significant traffic increase after setting user Id

  • February 15, 2025
  • 2 replies
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Hi, all 

 

It’s been for a year since we have installed Amplitude to our service for the first time. 

We haven’t used user id feature, but, we’ve decided to start using it from now on. 

The thing is, after setting user id, New User traffic has been increased weirdly.

 

We set the user id feature on 13, Feb and we saw a 500% increase in new users before and after the setting. I went through documentation regarding how Amplitude count unique users, but I was not able to get any clue from it. 

 

Anyone has experienced similar situation? I would love to hear your valuable experience. 

Thank you all for your help in advance. 

 

Best Regards, 

Offmate.

Best answer by Saish Redkar

Hi ​@Offmate 

You might have to verify how your instrumentation code is assigning user ids for existing users vs new users since you already have users in the anonymous state in your system.

I suspect that Amplitude could be reassigning new device_id/ user_id pair for existing users causing the spike.

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Saish Redkar
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  • Saish Redkar
  • Expert
  • 1380 replies
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  • February 18, 2025

Hi ​@Offmate 

You might have to verify how your instrumentation code is assigning user ids for existing users vs new users since you already have users in the anonymous state in your system.

I suspect that Amplitude could be reassigning new device_id/ user_id pair for existing users causing the spike.

Saish | Connect with me on topmate.io/saish_redkar for supercharging your Amplitude skills
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  • RobertConry
  • Experienced
  • 13 replies
  • March 14, 2025

That kind of traffic spike usually happens when you start setting User ID for existing anonymous users. Amplitude merges past anonymous activity with the new identified user, which can make it look like there’s a sudden surge in events. Check your User Composition report to see if the increase is coming from merged users.

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