Aki Vehtari
avehtari.bsky.social
Aki Vehtari
@avehtari.bsky.social
Professor in computational Bayesian modeling at Aalto University, Finland. Bayesian Data Analysis 3rd ed, Regression and Other Stories, and Active Statistics co-author. #mcmc_stan and #arviz developer.

Web page https://users.aalto.fi/~ave/
Pinned
All three books I've co-authored are freely available online for non-commercial use:

- #Bayesian Data Analysis, 3rd ed (aka BDA3) at stat.columbia.edu/~gelman/book/

- #Regression and Other Stories at avehtari.github.io/ROS-Examples/

- Active Statistics at avehtari.github.io/ActiveStatis...
Sea view today close to my home
November 11, 2025 at 12:24 PM
Reposted by Aki Vehtari
I am hiring to my group in @ellisinstitute.fi and @aifunmcr.bsky.social, DL Dec 1

Topics: Multimodal foundation models, out-of-distribution deployable machine learning, collaborative machine learning

kaski-lab.com
Samuel Kaski
kaski-lab.com
November 7, 2025 at 11:06 AM
Reposted by Aki Vehtari
Applications for the PhD and MSc programs in statistics at UBC are now open!

📆 Deadline for PhD program is December 1st
📆 Deadline for MSc program is January 5th

The department covers all areas of statistics and we have a lot of momentum in Bayesian computation!
November 7, 2025 at 8:05 PM
Reposted by Aki Vehtari
New blog post: open-source software packages have surprising problems with the way they calculate weighted medians and other quantiles.

www.practicalsignificance.com/posts/weight...

#rstats #julialang
Weighted Quantile Weirdness and Bugs – Practical Significance
Computing quantiles is surprisingly complicated. It gets much weirder when you use weights, and popular software behaves in surprising ways that might trouble you.
www.practicalsignificance.com
November 5, 2025 at 4:30 PM
Reposted by Aki Vehtari
Newly released Stockholm Declaration recommends the following reforms to publishing:

1. Academia resumes control of publishing

2. Incentive systems to merit quality, not quantity

3. Independent fraud detection and prevention

4. Legislation and policies to protect science quality and integrity
Reformation of science publishing: the Stockholm Declaration | Royal Society Open Science
Science relies on integrity and trustworthiness. But scientists under career pressure are lured to purchase fake publications from ‘paper mills’ that use AI-generated data, text and image fabrication....
royalsocietypublishing.org
November 5, 2025 at 7:07 PM
This is exactly what I think when I see someone mentioning fixed effects model
How to fit a model without estimating any parameters?

Use a formula with
y ~ 0 + offset(...)
This works with any formula interface, and with #tidymodels this can be passed to add_model().

#rstats
November 5, 2025 at 6:37 PM
Reposted by Aki Vehtari
🧠⚙️ Interested in decision theory+cogsci meets AI? Want to create methods for rigorously designing & evaluating human-AI workflows?

I'm recruiting PhDs to work on:
🎯 Stat foundations of multi-agent collaboration
🌫️ Model uncertainty & meta-cognition
🔎 Interpretability
💬 LLMs in behavioral science
November 5, 2025 at 4:40 PM
Reposted by Aki Vehtari
Modeling and numerics: could we do something better? by Juho Timonen.
www.generable.com/post/modelin...
Modeling and numerics: could we do something better? | Juho Timonen | Generable
The issues of the traditional ODE solvers in probabilistic models
www.generable.com
November 5, 2025 at 5:07 PM
Reposted by Aki Vehtari
A little self promotion: Hai-Dang Dau (NUS) and I recently released this pre-print, which I'm not half proud of.
arxiv.org/abs/2510.07559

The main problem we solve in it is to construct importance weights for Markov chain Monte Carlo. We achieve it via a method we call harmonization by coupling.
October 20, 2025 at 4:10 PM
Now I'm also looking for a research software engineer to implement a pile of research results to R packages loo, posterior, bayesplot, projpred, priorsense, brms or/and Python packages ArviZ, Bambi and Kulprit. Apply by email with no specific deadline (see contact info at users.aalto.fi/~ave/)
I'm now also looking for a postdoc with strong Bayesian background and interest in developing Bayesian cross-validation theory, methods and software. Apply by email with no specific deadline (see contact information at users.aalto.fi/~ave/).

Others, please share
I'm looking for a doctoral student with Bayesian background to work on Bayesian workflow and cross-validation (see my publication list users.aalto.fi/~ave/publica... for my recent work) at Aalto University.

Apply through the ELLIS PhD program (dl October 31) ellis.eu/news/ellis-p...
November 3, 2025 at 11:13 AM
Reposted by Aki Vehtari
Great news from the Research Council of Finland: Tuomas Hytönen, Mikko Möttönen and Aki Vehtari have been selected as Academy Professors!

Warm congratulations!  💐

www.aka.fi/en/about-the... 
Research Council of Finland selects 15 new Academy Professors
The Research Council of Finland (RCF) has decided on funding for 15 new Academy Professorships for the period 1 January 2026–31 December 2031. The revised Academy Professorship funding scheme provides funding to researchers who are at the international forefront of their fields and whose research strives for scientific renewal and breakthroughs. The funding is designed to support s...
www.aka.fi
October 30, 2025 at 1:05 PM
5 mins ago
October 29, 2025 at 6:08 PM
I'm now also looking for a postdoc with strong Bayesian background and interest in developing Bayesian cross-validation theory, methods and software. Apply by email with no specific deadline (see contact information at users.aalto.fi/~ave/).

Others, please share
I'm looking for a doctoral student with Bayesian background to work on Bayesian workflow and cross-validation (see my publication list users.aalto.fi/~ave/publica... for my recent work) at Aalto University.

Apply through the ELLIS PhD program (dl October 31) ellis.eu/news/ellis-p...
ELLIS PhD Program: Call for Applications 2025
The ELLIS mission is to create a diverse European network that promotes research excellence and advances breakthroughs in AI, as well as a pan-European PhD program to educate the next generation of AI...
ellis.eu
October 29, 2025 at 2:37 PM
"Uncertainty in Bayesian leave-one-out cross-validation based model comparison" with Tuomas Sivula, Asael Alonzo Matamoros, and @mansmag.bsky.social, has been published in Bayesian Analysis doi.org/10.1214/25-B...

🧵 1/
Uncertainty in Bayesian Leave-One-Out Cross-Validation Based Model Comparison
It is useful to estimate the expected predictive performance of models planned to be used for prediction. We focus on leave-one-out cross-validation (LOO-CV), which has become a popular method for estimating predictive performance of Bayesian models. Given two models, we are interested in comparing the predictive performances and associated uncertainty, which can also be used to compute the probability of one model having better predictive performance than the other model. We study the properties of the Bayesian LOO-CV estimator and the related uncertainty quantification for the predictive performance difference, and analyse when a normal approximation of this uncertainty is well calibrated and whether taking into account higher moments could improve the approximation. We provide new results of the properties both theoretically in the linear regression case and empirically for hierarchical linear, latent linear, and spline models and discuss the challenges. We show that problematic cases include: comparing models with similar predictions, misspecified models, and small data. In these cases, there is a weak connection between the distributions of the LOO-CV estimator and its error. We show that that the problematic skewness of the error distribution for the difference, which occurs when the models make similar predictions, does not fade away when the data size grows to infinity in certain situations. Based on the results, we also provide some practical recommendations for the users of Bayesian LOO-CV for comparing predictive performance of models.
doi.org
October 28, 2025 at 1:39 PM
Reposted by Aki Vehtari
I'm looking for a doctoral student with Bayesian background to work on Bayesian workflow and cross-validation (see my publication list users.aalto.fi/~ave/publica... for my recent work) at Aalto University.

Apply through the ELLIS PhD program (dl October 31) ellis.eu/news/ellis-p...
ELLIS PhD Program: Call for Applications 2025
The ELLIS mission is to create a diverse European network that promotes research excellence and advances breakthroughs in AI, as well as a pan-European PhD program to educate the next generation of AI...
ellis.eu
October 6, 2025 at 9:28 AM
Reposted by Aki Vehtari
I am hiring PhD candidates to study the psychology of attention & technology use at @tilburg-university.bsky.social.

We're looking for motivated & curious scholars with expertise in cognitive psychology and statistics, and offer a friendly work environment with great terms & benefits.

tiu.nu/22989
October 23, 2025 at 3:04 PM
Reposted by Aki Vehtari
Interested in generative AI and computing education? We are looking for a PhD student to work on developing and studying tools that help novice programmers learn how to use generative AI at @csaalto.bsky.social. Find out more & apply here:
www.aalto.fi/en/open-posi...
PhD Position in Educational Tools for Teaching Generative AI Use to Novice Programmers | Aalto University
To apply, please prepare a cover letter detailing your interest and qualifications for the position, a CV, and degree & study transcripts. Applications will be reviewed on a rolling basis and the position will be filled as soon as a suitable candidate is found, so don't delay.
www.aalto.fi
October 22, 2025 at 2:09 PM
October 21, 2025 at 3:58 PM
Reposted by Aki Vehtari
Compositional data needs models that respect its constraints. I show how Dirichlet regression with GPs captures nonlinear environmental effects on plant communities while respecting the dependence of the data #rstats #dataviz ecogambler.netlify.app/blog/plant-c...
October 21, 2025 at 8:19 AM
Reposted by Aki Vehtari
Personalized dosing decisions beyond clinical trials: www.generable.com/post/improvi...
Improving dosing decisions | Jacqueline Buros | Generable
Using a multistate model and patient utilities to improve dosing decisions
www.generable.com
October 17, 2025 at 2:05 PM
Reposted by Aki Vehtari
#epinowcast 0.4.0 is out! This release has been a long time coming and contains work from many contributors. It includdes new features, better better and clearer documentation.

package.epinowcast.org/news/index.h...
Changelog
package.epinowcast.org
October 17, 2025 at 3:10 PM
Reposted by Aki Vehtari
Against Publishing: universonline.nl/nieuws/2025/...

Preprints are read, shared, and cited, yet still dismissed as incomplete until blessed by a publisher. I argue that the true measure of scholarship lies in open exchange, not in the industry’s gatekeeping of what counts as published.
October 14, 2025 at 9:16 AM
Reposted by Aki Vehtari
StanCon 2026, Uppsala, Sweden

We are thrilled to announce that StanCon 2026 will take place in Uppsala, Sweden, from August 17th to August 21st, 2026. The conference brings together researchers and practitioners passionate about Bayesian inference and probabilistic programming in one of Sweden’s…
StanCon 2026, Uppsala, Sweden
We are thrilled to announce that StanCon 2026 will take place in Uppsala, Sweden, from August 17th to August 21st, 2026. The conference brings together researchers and practitioners passionate about Bayesian inference and probabilistic programming in one of Sweden’s most historic and vibrant university cities. Attendees will enjoy a week of conference talks, workshops, and tutorials spanning both foundational methods and real-world applications.
blog.mc-stan.org
October 13, 2025 at 2:53 PM
Reposted by Aki Vehtari
note that in the ggplot2 the default pointsize is not a constant anymore

GeomPoint$default_aes
#> Aesthetic mapping:
#> * `shape` -> `from_theme(pointshape)`
#> * `colour` -> `from_theme(colour %||% ink)`
#> * `fill` -> `from_theme(fill %||% NA)`
#> * `size` -> `from_theme(pointsize)`
October 9, 2025 at 8:24 PM
Reposted by Aki Vehtari
Have you made a package on top of Stan? Do you have a cool trick to make Stan faster or create a fancy model? Would you like to make a guest post at Stan blog blog.mc-stan.org?

Send me a private message if you are interested (and note that with our current blog plan we don't have MathJax support)
The Stan Blog
blog.mc-stan.org
October 9, 2025 at 12:52 PM