# \#priors

**URL:** https://discourse.mc-stan.org/tag/priors/106.md

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## [Advice setting powerscaled priors for longitudinal betabinomial spline model](https://discourse.mc-stan.org/t/advice-setting-powerscaled-priors-for-longitudinal-betabinomial-spline-model/40732)

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**Author:** [@llewmills](https://discourse.mc-stan.org/u/llewmills)\
**Replies:** 10\
**Last updated:** [December 22, 2025, 7:08pm UTC](https://discourse.mc-stan.org/t/advice-setting-powerscaled-priors-for-longitudinal-betabinomial-spline-model/40732 "2025-12-22T19:08:56Z")

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I am modeling heroin use during the first year of opioid agonist treatment based on different levels of amphetamine use at start of treatment. I am using brms. Days of amphetamine use at baseline in the 28 days prior to …

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## [Regularized Horseshoe Prior for Poisson Regression with Offset](https://discourse.mc-stan.org/t/regularized-horseshoe-prior-for-poisson-regression-with-offset/40646)

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**Author:** [@nkreimer](https://discourse.mc-stan.org/u/nkreimer)\
**Replies:** 1\
**Last updated:** [November 20, 2025, 11:15pm UTC](https://discourse.mc-stan.org/t/regularized-horseshoe-prior-for-poisson-regression-with-offset/40646 "2025-11-20T23:15:54Z")

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I am trying to follow Piironen and Vehtari (2017) to implement a regularized horseshoe prior for a Poisson regression model with an offset. I am modelling the number of events in a location with population size as an off…

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## [Directly using contrasts as predictors in brms to get BFs - a valid approach?](https://discourse.mc-stan.org/t/directly-using-contrasts-as-predictors-in-brms-to-get-bfs-a-valid-approach/40371)

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**Author:** [@Lukas\_Dorsch](https://discourse.mc-stan.org/u/Lukas_Dorsch)\
**Replies:** 0\
**Last updated:** [September 16, 2025, 5:45pm UTC](https://discourse.mc-stan.org/t/directly-using-contrasts-as-predictors-in-brms-to-get-bfs-a-valid-approach/40371 "2025-09-16T17:45:27Z")

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Hi everyone. I study Psychology and plan to do some Bayesian contrast analyses (e.g. a 2x2 interaction contrast within a larger 2x3 design). I’ve read through some documentation of bayestestR and brms as well as some in…

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## [Can \`powerscale()\` be used to obtain prior draws?](https://discourse.mc-stan.org/t/can-powerscale-be-used-to-obtain-prior-draws/40317)

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**Author:** [@mattansb](https://discourse.mc-stan.org/u/mattansb)\
**Replies:** 2\
**Last updated:** [September 3, 2025, 7:48am UTC](https://discourse.mc-stan.org/t/can-powerscale-be-used-to-obtain-prior-draws/40317 "2025-09-03T07:48:24Z")

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In {bayestestR} we use prior draws together with posterior draws to estimate Savage-Dickey ratios, Order restricted Bayes factors, and support intervals. Currently, the way to do this, for example with a brmsfit object …

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## [Difference between brms \`unupdate()\` and setting priors manually for calculating Bayes Factors](https://discourse.mc-stan.org/t/difference-between-brms-unupdate-and-setting-priors-manually-for-calculating-bayes-factors/39874)

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**Author:** [@Alex-A14](https://discourse.mc-stan.org/u/Alex-A14)\
**Replies:** 2\
**Last updated:** [July 11, 2025, 3:52pm UTC](https://discourse.mc-stan.org/t/difference-between-brms-unupdate-and-setting-priors-manually-for-calculating-bayes-factors/39874 "2025-07-11T15:52:10Z")

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Hello, I’m calculating Bayes Factors for a brms model using bayesfactor\_parameters() and I get wildly different results when manually setting a normal distribution, versus when I allow the function to calculate it’s own…

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## [R2D2M2 prior and monotonic predictors](https://discourse.mc-stan.org/t/r2d2m2-prior-and-monotonic-predictors/36712)

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**Author:** [@Xavier\_La\_Rochelle](https://discourse.mc-stan.org/u/Xavier_La_Rochelle)\
**Replies:** 4\
**Last updated:** [June 21, 2025, 12:25pm UTC](https://discourse.mc-stan.org/t/r2d2m2-prior-and-monotonic-predictors/36712 "2025-06-21T12:25:16Z")

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I’m currently trying to formulate a multilevel model which contains 46 ordinal predictors and 93 regression terms overall. The high number of predictors warranted the use of a shrinkage prior and I was considering the R2…

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## [Choice of regularizing prior for the cumulative probit](https://discourse.mc-stan.org/t/choice-of-regularizing-prior-for-the-cumulative-probit/39582)

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**Author:** [@epkanol](https://discourse.mc-stan.org/u/epkanol)\
**Replies:** 4\
**Last updated:** [May 28, 2025, 8:05pm UTC](https://discourse.mc-stan.org/t/choice-of-regularizing-prior-for-the-cumulative-probit/39582 "2025-05-28T20:05:47Z")

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I have a situation where I have a 7-item Likert scale, where each respondent answers 6 questions on the same subject (TAM). Essentially the same setup as in this question: Partial pooling on ordinal response (cumulative…

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## [Boundaries per coefficient in brms](https://discourse.mc-stan.org/t/boundaries-per-coefficient-in-brms/39554)

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**Author:** [@mariobecerra](https://discourse.mc-stan.org/u/mariobecerra)\
**Replies:** 1\
**Last updated:** [May 22, 2025, 3:54pm UTC](https://discourse.mc-stan.org/t/boundaries-per-coefficient-in-brms/39554 "2025-05-22T15:54:52Z")

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I’m trying to set individual boundaries for each coefficient in the prior when fitting a linear model in brms. Here’s the code to simulate a dataset with 3 groups, where each group comes from a truncated normal distribu…

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## [Homogeneity of variance / homoscedasticity in Bayesian models](https://discourse.mc-stan.org/t/homogeneity-of-variance-homoscedasticity-in-bayesian-models/39538)

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**Author:** [@lukaseamus](https://discourse.mc-stan.org/u/lukaseamus)\
**Replies:** 5\
**Last updated:** [May 21, 2025, 4:43pm UTC](https://discourse.mc-stan.org/t/homogeneity-of-variance-homoscedasticity-in-bayesian-models/39538 "2025-05-21T16:43:54Z")

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Homogeneity of variance, also homoscedasticity, is a major modelling assumption, which when using maximum likelihood is tested based on residual plots, i.e. is learned from the data. If data are heteroscedastic, variance…

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## [Understanding cumulative probit ordinal priors](https://discourse.mc-stan.org/t/understanding-cumulative-probit-ordinal-priors/39416)

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**Author:** [@epkanol](https://discourse.mc-stan.org/u/epkanol)\
**Replies:** 2\
**Last updated:** [May 4, 2025, 1:51pm UTC](https://discourse.mc-stan.org/t/understanding-cumulative-probit-ordinal-priors/39416 "2025-05-04T13:51:28Z")

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As I am dealing with 7-item Likert data, I am trying to get my hand around cumulative probit models, and how they are implemented in brms I don’t have much prior information, so I planned to use the technique mentioned …

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## [Brms - set priors correctly for group effects](https://discourse.mc-stan.org/t/brms-set-priors-correctly-for-group-effects/31972)

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**Author:** [@thor6](https://discourse.mc-stan.org/u/thor6)\
**Replies:** 11\
**Last updated:** [December 10, 2024, 4:22pm UTC](https://discourse.mc-stan.org/t/brms-set-priors-correctly-for-group-effects/31972 "2024-12-10T16:22:51Z")

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I’m trying to understand how is the best way to set priors for group effects taking into account the variation between them, because I’m a bit confused about the class = “sd”. I have the following case where I want to es…

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## [Inequality constraints on linear combinations of parameters](https://discourse.mc-stan.org/t/inequality-constraints-on-linear-combinations-of-parameters/39077)

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**Author:** [@Arne\_Henningsen](https://discourse.mc-stan.org/u/Arne_Henningsen)\
**Replies:** 21\
**Last updated:** [March 23, 2025, 3:50pm UTC](https://discourse.mc-stan.org/t/inequality-constraints-on-linear-combinations-of-parameters/39077 "2025-03-23T15:50:20Z")

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Hi I would like to use Stan to estimate a regression model with inequality constraints on linear combinations of parameters. Here is a simplified example, a quadratic function: y\_i = \\beta\_0 + \\beta\_1 x\_{1,i} + \\beta\_2…

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## [Choosing priors in brms for interaction term and covariance matrix](https://discourse.mc-stan.org/t/choosing-priors-in-brms-for-interaction-term-and-covariance-matrix/39095)

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**Author:** [@ShabnamShadloo](https://discourse.mc-stan.org/u/ShabnamShadloo)\
**Replies:** 2\
**Last updated:** [March 19, 2025, 9:23pm UTC](https://discourse.mc-stan.org/t/choosing-priors-in-brms-for-interaction-term-and-covariance-matrix/39095 "2025-03-19T21:23:43Z")

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I am trying to model a proportional zero inflated response variable. The number of sample sizes are not the same throughout. I have a covariance matrix resulted from a phylogenetic tree as a random effect. I have a cate…

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## [Priors Not Having an Effect on Posterior Samples](https://discourse.mc-stan.org/t/priors-not-having-an-effect-on-posterior-samples/39058)

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**Author:** [@Stat\_Guy](https://discourse.mc-stan.org/u/Stat_Guy)\
**Replies:** 5\
**Last updated:** [March 17, 2025, 1:26am UTC](https://discourse.mc-stan.org/t/priors-not-having-an-effect-on-posterior-samples/39058 "2025-03-17T01:26:38Z")

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Hi Stan community, I’m new using this program. I’m having trouble getting my priors to influence the posterior samples in my Stan model. I’ve tried tightening the priors and reducing the number of samples to minimize th…

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## [Priors for multinomial logistic regression in brms](https://discourse.mc-stan.org/t/priors-for-multinomial-logistic-regression-in-brms/38424)

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**Author:** [@FeliOtte](https://discourse.mc-stan.org/u/FeliOtte)\
**Replies:** 5\
**Last updated:** [February 13, 2025, 10:31am UTC](https://discourse.mc-stan.org/t/priors-for-multinomial-logistic-regression-in-brms/38424 "2025-02-13T10:31:41Z")

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Hi everyone, I’m trying to run a multinomial logistic regression model in brms and am struggling to set priors. I only have 3000 data points (actually a lot of data in my field, but not for statistical modelling), so I’…

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## [ARR2: A new prior for autoregressions](https://discourse.mc-stan.org/t/arr2-a-new-prior-for-autoregressions/35363)

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**Author:** [@n-kall](https://discourse.mc-stan.org/u/n-kall)\
**Replies:** 5\
**Last updated:** [February 7, 2025, 1:40pm UTC](https://discourse.mc-stan.org/t/arr2-a-new-prior-for-autoregressions/35363 "2025-02-07T13:40:23Z")

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Hi all, We (@davkoh, @n-kall, @yannmclatchie, @avehtari) have a new preprint on arXiv presenting the ARR2 prior (a prior on R2 for autoregressive models). An explicit prior on R2 controls the tendency for the prior on …

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## [Proper setup of priors for a drift difusion model](https://discourse.mc-stan.org/t/proper-setup-of-priors-for-a-drift-difusion-model/38502)

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**Author:** [@pbranc](https://discourse.mc-stan.org/u/pbranc)\
**Replies:** 3\
**Last updated:** [January 16, 2025, 6:56pm UTC](https://discourse.mc-stan.org/t/proper-setup-of-priors-for-a-drift-difusion-model/38502 "2025-01-16T18:56:17Z")

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I am working an adrift diffusion model and am running into an issue setting up priors.I recently made a post where, with the help of another forum user, I’ve decided my model formula should look like this: formula \<- b…

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## [BRMS: Priors in multi-variate hierarchical models](https://discourse.mc-stan.org/t/brms-priors-in-multi-variate-hierarchical-models/36981)

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**Author:** [@Alessiolo](https://discourse.mc-stan.org/u/Alessiolo)\
**Replies:** 4\
**Last updated:** [November 17, 2024, 11:39am UTC](https://discourse.mc-stan.org/t/brms-priors-in-multi-variate-hierarchical-models/36981 "2024-11-17T11:39:41Z")

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Heya, I am trying to fit a multivariate (multiple-outcome) hierarchical model in brms. The data and most of the code was taken from: https://cran.r-project.org/web/packages/brms/vignettes/brms\_multivariate.html#more-c…

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## [Advice on priorsense and splines](https://discourse.mc-stan.org/t/advice-on-priorsense-and-splines/29945)

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**Author:** [@andymilne](https://discourse.mc-stan.org/u/andymilne)\
**Replies:** 12\
**Last updated:** [November 11, 2024, 7:08pm UTC](https://discourse.mc-stan.org/t/advice-on-priorsense-and-splines/29945 "2024-11-11T19:08:41Z")

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I am running a model in brms with a couple of smooth terms. I thought it would be interesting to use the priorsense package. It is finding several prior-data conflicts for the various smooth terms used in brms. Changing …

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## [Need help with priorsense::powerscale complaining about infinite or NA values](https://discourse.mc-stan.org/t/need-help-with-priorsense-powerscale-complaining-about-infinite-or-na-values/37031)

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**Author:** [@karimn](https://discourse.mc-stan.org/u/karimn)\
**Replies:** 8\
**Last updated:** [October 29, 2024, 8:50pm UTC](https://discourse.mc-stan.org/t/need-help-with-priorsense-powerscale-complaining-about-infinite-or-na-values/37031 "2024-10-29T20:50:29Z")

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Hi, I’m using priorsense v1.0.3 and I’m getting an warning that is preventing me from getting a pareto-k for my powerscaling. I’m not sure how to find these problematic inputs. \> powerscale(fit, alpha = 0.9, component …

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## [Individual priors for coefficients in regression](https://discourse.mc-stan.org/t/individual-priors-for-coefficients-in-regression/36516)

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**Author:** [@Chr\_And](https://discourse.mc-stan.org/u/Chr_And)\
**Replies:** 3\
**Last updated:** [September 8, 2024, 5:04pm UTC](https://discourse.mc-stan.org/t/individual-priors-for-coefficients-in-regression/36516 "2024-09-08T17:04:29Z")

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I using rstan to estimate the average time use for different laboratory analysis. The model is T = a\*beta, where T is a vector of length N of time used per person in different projects, a is a NxA matrix with counts of …

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## [Setting weakly informative priors in psychology](https://discourse.mc-stan.org/t/setting-weakly-informative-priors-in-psychology/36241)

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**Author:** [@max-kat](https://discourse.mc-stan.org/u/max-kat)\
**Replies:** 5\
**Last updated:** [August 15, 2024, 8:54pm UTC](https://discourse.mc-stan.org/t/setting-weakly-informative-priors-in-psychology/36241 "2024-08-15T20:54:01Z")

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Dear community, I am a psychologist and I recently followed the tutorial of Bürkner & Vuorre to analyze my Likert-scale data with an ordinal hierarchical regression. At first I did not specify any priors cause the study…

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## [Unsure what the prior is doing!](https://discourse.mc-stan.org/t/unsure-what-the-prior-is-doing/36074)

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**Author:** [@KleinGordon](https://discourse.mc-stan.org/u/KleinGordon)\
**Replies:** 3\
**Last updated:** [August 1, 2024, 12:46pm UTC](https://discourse.mc-stan.org/t/unsure-what-the-prior-is-doing/36074 "2024-08-01T12:46:42Z")

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I am trying to implement a stan model presented in “Statistical Rethinking” in Section 14.2, pg 438, R code 14.13. I’ve pasted the example code below: model\_code \<- ' data{ int N; int nc\_num\_missing; vector\[N\] …

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## [Help with Bayesian Modelling](https://discourse.mc-stan.org/t/help-with-bayesian-modelling/35668)

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**Author:** [@Patrick\_O\_Rourke](https://discourse.mc-stan.org/u/Patrick_O_Rourke)\
**Replies:** 6\
**Last updated:** [July 4, 2024, 9:26pm UTC](https://discourse.mc-stan.org/t/help-with-bayesian-modelling/35668 "2024-07-04T21:26:00Z")

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I am currently working on my postgraduate dissertation in statistics and I am using STAN extensively for this. My dissertation involves simulating a latent position model of chess games which estimates the ratings of the…

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## [Random walk in RStan](https://discourse.mc-stan.org/t/random-walk-in-rstan/34308)

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**Author:** [@lukaseamus](https://discourse.mc-stan.org/u/lukaseamus)\
**Replies:** 11\
**Last updated:** [June 12, 2024, 6:45am UTC](https://discourse.mc-stan.org/t/random-walk-in-rstan/34308 "2024-06-12T06:45:50Z")

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I recently attempted using random walk in the rethinking::ulam interface to RStan and failed. @richard\_mcelreath replied that ulam does not currently support random walk. Now I am looking into achieving this directly in…

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## [Problems with using Dirichlet Prior in Stan](https://discourse.mc-stan.org/t/problems-with-using-dirichlet-prior-in-stan/30798)

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**Author:** [@Emap](https://discourse.mc-stan.org/u/Emap)\
**Replies:** 1\
**Last updated:** [May 14, 2024, 7:22pm UTC](https://discourse.mc-stan.org/t/problems-with-using-dirichlet-prior-in-stan/30798 "2024-05-14T19:22:51Z")

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So I’m trying to fit a model through Stan which was initially working fine, however after adding the phi parameter and giving it a Dirichlet prior I’m getting a large amount of divergent samples and also some transitions…

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## [Reference prior specification for proportional odds model](https://discourse.mc-stan.org/t/reference-prior-specification-for-proportional-odds-model/35035)

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**Author:** [@cbayes](https://discourse.mc-stan.org/u/cbayes)\
**Replies:** 0\
**Last updated:** [May 6, 2024, 2:52am UTC](https://discourse.mc-stan.org/t/reference-prior-specification-for-proportional-odds-model/35035 "2024-05-06T02:52:33Z")

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Hi all, let’s say that one is interested in estimating a proportional odds ratio between a single binary exposure and an ordinal outcome with J categories using Bayesian inference: what is a standard form of a reference …

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## [Calculate intercept by centralising residuals instead of estimating intercept?](https://discourse.mc-stan.org/t/calculate-intercept-by-centralising-residuals-instead-of-estimating-intercept/34728)

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**Author:** [@Arne\_Henningsen](https://discourse.mc-stan.org/u/Arne_Henningsen)\
**Replies:** 2\
**Last updated:** [April 11, 2024, 11:02am UTC](https://discourse.mc-stan.org/t/calculate-intercept-by-centralising-residuals-instead-of-estimating-intercept/34728 "2024-04-11T11:02:08Z")

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I want to fit a model, where I do not care about the intercept and I have no idea about its potential value so that it is difficult to specify a reasonable prior for the intercept. I wonder whether it is best (a) to use …

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## [How to address uncertainty of point estimates in priors](https://discourse.mc-stan.org/t/how-to-address-uncertainty-of-point-estimates-in-priors/34677)

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**Author:** [@stanbeginner](https://discourse.mc-stan.org/u/stanbeginner)\
**Replies:** 3\
**Last updated:** [April 4, 2024, 7:11am UTC](https://discourse.mc-stan.org/t/how-to-address-uncertainty-of-point-estimates-in-priors/34677 "2024-04-04T07:11:58Z")

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When using posterior information from a pilot study to inform priors in my main experiment, I have been advised to “broaden” the SDs of the point estimates, e.g., by using twice the SD for the prior compared with the pos…

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## [Choose prior based on hypothesis?](https://discourse.mc-stan.org/t/choose-prior-based-on-hypothesis/34616)

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**Author:** [@stanbeginner](https://discourse.mc-stan.org/u/stanbeginner)\
**Replies:** 2\
**Last updated:** [April 1, 2024, 2:23pm UTC](https://discourse.mc-stan.org/t/choose-prior-based-on-hypothesis/34616 "2024-04-01T14:23:34Z")

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Hello, I have some data from a pilot study and plan to analyze the (new) data of the main experiment. My idea was to use the posteriors of the pilot analyses as priors for the Bayesian analyses of the main experiment. Ho…

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