What do chickenpox and shingles have to do with your brain? This week, we dig into two 2025 headline-grabbing studies that link the shingles shot to lower dementia rates. We start in Wales, where a birthday cutoff turned into the perfect natural experiment, and end in the U.S. with a multi-million-person megastudy. Featuring bias-variance Goldilockses, Fozzy-the-Bear regression discontinuities, a Barbie-versus-Oppenheimer showdown for propensity scores – and the hottest rebrand of inverse-probability weighting you’ll ever hear.

Statistical topics

• Absolute vs. relative risk
• Bias–variance tradeoff
• Causal inference
• Censoring
• Confounding
• Fuzzy regression discontinuity design
• Healthy-user bias
• Inverse probability of treatment weighting (IPTW)
• Longitudinal study
• Natural experiment
• Negative controls
• Optimal bandwidth
• Propensity scores
• Selection bias
• Subgroup analysis
• Triangular kernel weights

Methodological morals

• “Propensity scores are the lipstick you put on observational pigs.”
• “Natural experiments are a hot flirtation date with causality.”

References

• Eyting M, Xie M, Michalik F, Heß S, Chung S, Geldsetzer P. A natural experiment on the effect of herpes zoster vaccination on dementia. Nature. 2025 May;641(8062):438-446. doi: 10.1038/s41586-025-08800-x. Epub 2025 Apr 2. PMID: 40175543; PMCID: PMC12058522.
• Polisky V, Littmann M, Triastcyn A, et al. Varicella-zoster virus reactivation and the risk of dementia. Nat Med. Published online October 6, 2025. doi:10.1038/s41591-025-03972-5
• Sainani KL. Propensity scores: uses and limitations. PM&R 2012; 4:693-97.

Detailed Show Notes Page

Kristin and Regina’s online courses: 

Demystifying Data: A Modern Approach to Statistical Understanding  

Clinical Trials: Design, Strategy, and Analysis 

Medical Statistics Certificate Program  

Writing in the Sciences 

Epidemiology and Clinical Research Graduate Certificate Program 

Programs that we teach in:

Epidemiology and Clinical Research Graduate Certificate Program 

Find us on:

Kristin -  LinkedIn & Twitter/X

Regina - LinkedIn & ReginaNuzzo.com

• (00:00) - Intro and first gratuitous mention of sex
• (03:56) - What are shingles, chickenpox, and the vaccines against them?
• (12:30) - Fun facts about the varicella zoster and herpes viruses
• (18:00) - A natural experiment in Wales
• (21:54) - What is the Goldilocks optimal bandwidth?
• (26:17) - Fuzzy regression discontinuity design demystified
• (32:43) - Shingles vaccine vs dementia showdown
• (34:13) - Absolute risk reduction paradox
• (37:44) - Effects for men and women differ
• (41:07) - A giant longitudinal study
• (47:51) - Propensity scores demystified via Barbie and Oppenheimer
• (53:55) - Using propensity scores to make matches
• (58:08) - Inverse probability of treatment weighting demystified via more Barbenheimer
• (01:02:27) - Attempts to rename IPTW for TikTok
• (01:05:59) - Longitudinal study results
• (01:10:00) - Smooch ratings and methodological morals: pigs and hot dates