Questions tagged [overdispersion]

Overdispersion is when there is greater variability than there 'ought' to be in the data. Eg, the variance of counts is often greater than the mean, whereas the variance of a Poisson should equal the mean.

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Help interpreting output from glmmTMB and Ben Bolker's overdispersion function

Just wondering if anyone can help with interpreting the output from Ben Bolker's over-dispersion function (please see link below): https://bbolker.github.io/mixedmodels-misc/glmmFAQ.htmlhttps://...
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Probability of multiple parasites given the presence of one parasite

I am trying to simulate parasitic plants that infect trees. I would like to test wether the presence of one parasite makes it more likely that the tree has more than one parasite. The number of trees ...
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Unadjusted rates vs. observed rates?

In poisson and negative binomial rate models, should the observed rate be the same as the unadjusted rate (in model with only 1 variable)? Should you report these unadjusted rates from a model with ...
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DHARMA to detect overdispersion in negative binomial

I'm new to negative binomial GLMMs and still trying to get a hold of checking my residuals. DHARMa has been a huge help, but I still am having some inconsistent results. I am looking at three groups ...
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Dealing with Overdispersed Negative Binomial using glmmTMB

I'm new to the world of statistical modeling, but I was wondering if anyone had any input on how to handle overdispersed negative binomial data? I'm working on modeling bat activity as a response ...
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Overdispersed poisson-distribution and offset --> standard errors?

I am modeling count data using R and doing a fixed-effects/random-effects model and thus limited on functions and therefore cannot use a quasipoisson model or negative binomial distribution, but ...
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`dispersiontest()` estimates dispersion too small

I am using dispersiontest(fit, trafo=2) from the AER package in R to see if my data is overdispersed and what the dispersion parameter $\alpha$ is. Since I use <...
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GAM model residuals

I have a huge problem with my model. I did a GAM Model with negative binomial distribution (with mgcv library). Now I want to do an overdispersion test but I think that this doesn't exist for ...
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Is my overdispersion too large in this quasibinomial model?

I have used a quasibinomial model on my data, but my overdispersion coefficient seems to be too large with a value of 40.78776. ...
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How do I carry out a significance test with Tarone's Z-statistic?

Context In this blog the author suggests using Tarone's Z-statistic to test for overdispersion in a binomial model to determine whether or not it is necessary to use a beta-binomial model instead. In ...
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Overdispersion tests from DHARMa and sjstats: conflicting results?

I ran some models for my count data, and did some diagnostics to check for overdispersion. Here is a dharma graph, which as I understand, indicates no overdispersion. And this is the result I get ...
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Adjusting for clustering and overdispersion in count models

My question is specific to the estimation of glm's and correcting for 'clustering' in a quasi-experiment (difference-in-differences). My outcome is counts of crimes....
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DHARMa diagnostics: testDispersion and testZeroInflation interpretation

I have been analyzing count data using Poisson distribution in glmmTMB, and just ran some DHARMA diagnostics. However, there don't seem to be a lot of help online on how to interpret the results. Does ...
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QQ plot doesn't look great - maybe quasipoisson with random effects?

I have several glmer()-models that look like this: ...
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Is there a way to address overdispersion in a gls model?

I have autocorrelated data that show a positive linear increase. When I model them using gls, I think the summary shows overdispersion. When using GLMM etc I'd change error structure, but I don't ...
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Is there a single discrete distribution that handles over and under dispersion? [duplicate]

I have some count data I am trying to model. The variance is very close to the mean, so the Poisson distribution for the entire data set seems like a good starting point. I have done and it seems to ...
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Poisson model appears overdispersed, but usual recommended approaches don't improve fit

Summary: I am trying to model some count data. I initially attempted to fit a poisson GLM, but diagnostics appear to indicate overdispersion. I have tried several different recommended remedies but ...
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the importance of estimating correctly the dispersion parameter in hypothesis testing

I am reading an article, which is related to negative binomial model in estimating the dispersion parameter. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0081415 The author ...
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MLE for Overdispersed Poisson

I searched for a while on Google and this website for an answer to this question. I have an overdispersed Poisson distribution and a "hand-wavy" proof is giving me problems. Below is the information ...
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Choosing the optimal theta / dispersion parameter for negative binomial regression (glm / glm.nb) in R

I am applying a negative binomial regression to my data in R. For this, I use the package MASS and have two different ways to calculate it: ...
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Overdispersed proportion test?

I am comparing proportions in genomic data. In particular, I compare two populations, A and B, and I look at genetic sites. In other words, I have N contingency tables ...
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Are over-dispersion tests in GLMs actually *useful*?

The phenomenon of 'over-dispersion' in a GLM arises whenever we use a model that restricts the variance of the response variable, and the data exhibits greater variance than the model restriction ...
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Residual Deviance and degrees of freedom - Negative Binomial Distribution

I am trying to model count data using python's statsmodels module (Beer's sold at a football stadium as function of visitors, "tilskuer", and weather data). ...
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How to perform over-dispersion test where null is quasi-Poisson

If I understand correctly, a quasi Poisson regression assumes roughly that $$ \mbox{E}\left[y\left|x\right.\right] = \exp{\left(x^{\top}\beta\right)}, \quad \mbox{VAR}\left(y\left|x\right.\right) = \...
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Overdispersion problem in a quasi-binomial GLM (for proportional data)

Below is the summary of a GLM I built for a response variable which is proportional (derived from count data). My only predictor is a continuous one (environmental measurement). And my sample size is ...
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Control for over-dispersion. Why do this: take natural log of metric, exponentiate, rank, remove top and bottom 10%

I'm looking at some NHS healthcare data on the number of deaths in England The measure i'm looking at is called the SHMI - it's simply: The number of observed deaths at a hospital / The expected ...
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XGBoost Poisson Objective Function When Data is Over-dispersed [closed]

I am modeling very over-dispersed count data with the goal of prediction. The data is not zero inflated (there are no zeros), but there are a lot of values of 1. ...
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Interpreting Quasi-Linear Regression Predictions

I know that for a simple linear regression the predictions are distributed like: $$y_i\, |\, x_i\, \sim\, \mathcal{N}\big(\widehat{\beta}_0+\widehat{\beta}_1\, x_i,\ \sigma^2\big)$$ $$\text{where: } \...
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Can you use glmmTMB to simultaneously model offsets and zero-inflation?

I'm currently modelling microbial data, with multiple samples and groups of samples. Two problems arise with my data: 1) The data is zero-inflated and dispersed (large variation); 2) Each sample has a ...
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How to recalculate variance/covariance matrix adding a overdispersion term in glmm?

When models suffer from overdispersion, a solution is to calculate the dispersion parameter (using c_hat or dispersion_glmer in R for example). Then multiply the variance/covarianze matrix of ...
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Adding an observation level random term messes up residuals vs fitted plot. Why?

I run a mixed effects generalized model for proportional data (response variable). I used binomial family and logit link function. I suffered from overdispersion so I added an observation level random ...
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GLM: binomial(logit) with weights=tested

I am trying to decipher what this GLM means for a test: glm(proportion.correct ~ dose*drug, family=binomial(logit), weights=tested) The experiment looks at the ...
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binary logistic regression models with correction on over-dispersion [closed]

When using Generalized Lienar Models in SPSS (dist: binomial, link: logit, r/n) how to overdispersion correction? In otherwords how to apply binary logistic regression models with correction on over-...
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Name of the test for over/under dispersion

When testing for over or underdispersion in a count variable there is a test (available for example in the glm.nb() function in the ...
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Residual deviance and interactions in quasi-poisson model and negative binomial model?

My data was over-dispersed (dispersion coefficient over 5), so I have fitted both the quasi-poisson model and the negative binomial model. I notice that the regression coefficients are almost the same,...
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Quasi Poisson model quasi-likelihood

Wikipedia (https://en.wikipedia.org/wiki/Variance_function) gives this as the general definition of quasi-(log)-likelihood: $$ Q(\mu,y)= \int _{y}^{\mu} \frac{y-t}{\sigma^2 V(t)} dt$$ In the case of ...
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Changing from Poisson to NB distribution fixes overdispersion and improves model

I have count data (seconds of behaviour, n=145) with one explanatory categorical variable (Treatment) and 2 interaction terms (Sex and Time). I began by running a GLM with Poisson distribution: <...
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Non-constant overdispersion (overdispersion parameter depending on covariate) in negative binomial regression

In a negative binomial regression (daily number of cases of a disease over time), after controlling for several factors and experimenting with several functional forms, my results still don't look ...
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Validation of poisson GLMM corrected for overdispersion with OLRE

I am fitting a random effects model to some ecological count data (abundance). The aim is to analyse the effect of influence of chemical pest control on insect population. I run the analysis on ...
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Investigate overdispersion in a plot for a poisson regression

I run a poisson regression by hand and now want to investigate plots whether overdispersion might be a problem or not. What should I plot against each other?
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Overdispersion. Is it ever appropriate to have a non-linear variance correction from Poisson to a Quasi-Poisson?

This is my first Cross Validated post. Please let me know if this is too specific/innapropriate for the site. I am building a Poisson/quasiposson model with one predictor. The response variable ...
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Overdispersion in Count Data (Poisson model)

There is more than one solution for the problem of overdispersed count data. One is to use a quasipoisson model. One is to use a negative binomial model. One is to use a mixed-level model with subject-...
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how to analyze overdispersed binary data

In my substantive research, I often use dichotomous scoring (1 correct, 0 wrong) for my tests (tests with $15~yes/no$ items). My goal is often to compare the the proportion of correct answers to all ...
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glmm with proportion data and high overdispersion

I have data of time budget behaviors in form of relative frequencies (Nr specific behavior/nr of total behavior observed). The best would be to perform a multinomial analysis, but it seems too hard to ...
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How to intuitively explain the maths behind overdispersion?

I understand that overdispersion indicates extra, unexplained variation in the response than would be expected based on the statistical model of choice. And that the residual deviance is a measure of ...
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Causes for Underdispersion in Poisson Regression

I am working with count data (number of pregnancies per woman), and using glm Poisson (log-link) to model determinants of the former count variable. From simple descriptives I observe that my data ...
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Quasi-likelihood/Quasi Poisson

I'm facing this new concept: the quasi likelihood. I'm looking for some clear explanation of what it is. I have a very basic knowledge about this, so I need to go step by step very slowly. I ...
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Overdispersion in a binomial GLMER model

I'm having trouble accounting for overdispersion in a binomial GLMER (lme4 package) - I'd read through other posts on the topic but haven't found anything that solves my problem. I tried adding an ...
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The mean and variance of Poisson distribution are equal

This blog claims that the fact that the mean and variance of Poisson distribution are equal can cause problem. Could you please elaborate why this is the limitation and how it can affect models?