# |proj(x-g)-x| parameter

**URL:** https://simpeg.discourse.group/t/proj-x-g-x-parameter/109
**Category:** Getting Started
**Created:** [May 25, 2020, 8:46am UTC](https://simpeg.discourse.group/t/proj-x-g-x-parameter/109 "2020-05-25T08:46:22Z")
**Posts on this page:** 2
**Page:** 1

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### Author: ![Alexey](https://avatars.discourse-cdn.com/v4/letter/a/47e85d/32.png) [@Alexey](https://simpeg.discourse.group/u/Alexey)
#### Post date: [May 25, 2020, 8:46am UTC](https://simpeg.discourse.group/t/proj-x-g-x-parameter/109/1 "2020-05-25T08:46:22Z")

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Hi,

What characterizes value **|proj(x-g)-x|** in each iteration?

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### Author: ![thibaut.astic](https://yyz2.discourse-cdn.com/free1/user_avatar/simpeg.discourse.group/thibaut.astic/32/109_2.png) [@thibaut.astic](https://simpeg.discourse.group/u/thibaut.astic)
#### Post date: [December 7, 2020, 5:52pm UTC](https://simpeg.discourse.group/t/proj-x-g-x-parameter/109/2 "2020-12-07T17:52:46Z")

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Hi Alexey,  
Sorry for the late answer,  
I believe it is the norm of the update to the model. It checks if the model changes significantly enough.  
x: your model  
g: the gradient or `\delta m`  
proj: projection (if you use lower and upper bounds)