# Shared standard deviation in pymc.LKJCholeskyCov

**URL:** <https://discourse.pymc.io/t/shared-standard-deviation-in-pymc-lkjcholeskycov/13556>\
**Category:** v5\
**Tags:** modeling\
**Created:** [January 1, 2024, 4:43pm UTC](https://discourse.pymc.io/t/shared-standard-deviation-in-pymc-lkjcholeskycov/13556 "2024-01-01T16:43:28Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![dreeves](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/dreeves/32/6756_2.png) [@dreeves](https://discourse.pymc.io/u/dreeves)\
**Post date:** [January 1, 2024, 4:43pm UTC](https://discourse.pymc.io/t/shared-standard-deviation-in-pymc-lkjcholeskycov/13556/1 "2024-01-01T16:43:28Z")

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Hello,

I’m interested in using `pymc.LKJCholeskyCov` to estimate the covariance matrix for varying intercepts in a model that I am building. To use this function, we are required to provide the dimensionality of the covariance matrix `n` as well as a distribution of the standard deviations (using the `sd_dist` parameter).

What I would like to do for my model is use the assumption that the varying intercepts have the same standard deviation. As a result, the only source of covariance will be from the correlations amongst the intercepts. Is it possible to enforce such an assumption when using `pymc.LKJCholeskyCov`. I am aware that there is another option to use the `pymc.LKJCorr` prior instead which would allow me to define my covariance matrix in the way described above. However, I am having trouble getting the `pymc.LKJCorr` prior to work when using more than a few varying intercepts. In addition, I would like to use a non-centered parameterization for the intercepts.

Do you have any suggestions on how I can use `pymc.LKJCholeskyCov` in the way that I desire or will I need to do some manual work to have the intercepts share the same standard deviation?

Thank you for your help!

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**Author:** ![ricardoV94](https://yyz2.discourse-cdn.com/flex036/user_avatar/discourse.pymc.io/ricardov94/32/5775_2.png) [@ricardoV94](https://discourse.pymc.io/u/ricardoV94)\
**Post date:** [January 2, 2024, 10:06am UTC](https://discourse.pymc.io/t/shared-standard-deviation-in-pymc-lkjcholeskycov/13556/2 "2024-01-02T10:06:13Z")

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I think that might be a degenerate condition the way the LKJCholeskyCov is defined but maybe not. Perhaps @aseyboldt can provide more clear advice.