ArticleEcology and evolution2022
Fixed or random? On the reliability of mixed-effects models for a small number of levels in grouping variables.
Article in Ecology and evolution, 2022. The graph could read no effect estimate from its abstract, so it casts no vote on the map. Cited by 33 papers, 1 of them a synthesis that pooled it.
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Who cites it
33 citing papers in PubMed, 1 synthesis or guideline pooled it.
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- The Implications of Ungulate Population Dynamics for Conservation and Recovery Efforts in the Middle East.Ecology and evolution · 2026Article
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- Fine-scale spatial variation in eastern grey squirrel melanism across a complex urban landscape.The Journal of animal ecology · 2026Article
- Simulated Range Expansion Suggests Rapid Change in Biotic Resistance to a Range-Shifting Competitor.Ecology and evolution · 2026Article
- When to Take a Break: Comparing Effects of Systematic Short (Pomodoro) and Self-Regulated Breaks on Subjective Experience and Actual Learning.Behavioral sciences (Basel, Switzerland) · 2026Article
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- Therapist Memory for Treatment Contents: Implications for Patient Outcomes.Research square · 2026Article
- Rotational Grazing Supports Coexistence of the Endangered Pygmy Bluetongue and Livestock on a Productive Agricultural Property.Ecology and evolution · 2025Article
- Freeze-tolerant frogs accumulate cryoprotectants using photoperiod: A potential ecological trap.The Journal of animal ecology · 2025Article
- Causal association of dietary intake habits and telomere lengths: A Mendelian randomization study.Asia Pacific journal of clinical nutrition · 2025Article
- Acute phase responses in clinically healthy multiparous Holsteins with and without calcium dysregulation during the early postpartum period.Journal of dairy science · 2025Article
- Volitional spatial attention is lateralized in crows.Proceedings. Biological sciences · 2025Article
- Methods for meta-analysis and meta-regression of binomial data: concepts and tutorial with Stata command metapreg.Archives of public health = Archives belges de sante publique · 2024Article
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Authors and funding
3 authors.
Funding
No grant is acknowledged in the PubMed record.
Abstract
Biological data are often intrinsically hierarchical (e.g., species from different genera, plants within different mountain regions), which made mixed-effects models a common analysis tool in ecology and evolution because they can account for the non-independence. Many questions around their practical applications are solved but one is still debated: Should we treat a grouping variable with a low number of levels as a random or fixed effect? In such situations, the variance estimate of the random effect can be imprecise, but it is unknown if this affects statistical power and type I error rates of the fixed effects of interest. Here, we analyzed the consequences of treating a grouping variable with 2-8 levels as fixed or random effect in correctly specified and alternative models (under- or overparametrized models). We calculated type I error rates and statistical power for all-model specifications and quantified the influences of study design on these quantities. We found no influence of model choice on type I error rate and power on the population-level effect (slope) for random intercept-only models. However, with varying intercepts and slopes in the data-generating process, using a random slope and intercept model, and switching to a fixed-effects model, in case of a singular fit, avoids overconfidence in the results. Additionally, the number and difference between levels strongly influences power and type I error. We conclude that inferring the correct random-effect structure is of great importance to obtain correct type I error rates. We encourage to start with a mixed-effects model independent of the number of levels in the grouping variable and switch to a fixed-effects model only in case of a singular fit. With these recommendations, we allow for more informative choices about study design and data analysis and make ecological inference with mixed-effects models more robust for small number of levels.
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