# 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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26 views

### Modelling overdispersed rate data using a negative-binomial distribution

A quick overview of the analysis I'm wanting to do: I am wanting to analyze the relationship between habitat factors and the capture of my research species over a network of traps, in order to be able ...
43 views

### How to correct pattern in residuals when using a negative binomial distribution? Is it due to overdispersion?

I have a large dataset with the count data and I computed the following full model (covariates are all scaled) : ...
21 views

### How to work with continuous overdispered data?

I have troubles with analysis of my data. I analyze cross-sectional data about users activities and spendings from mobile game . I have paying and non-paying users, I need to explain what independent ...
47 views

### How to correct conditional Poisson standard errors for over-dispersion

I want to estimate a conditional negative binomial model, which an economist might call a negative binomial model with individual fixed effects. I use Python and statsmodels, which has a conditional ...
39 views

### Python statsmodels, handling over-dispersion for Poisson Regression

I have a Poisson model (displayed below), where my $\epsilon_e$ term is designed to handle over-dispersion. I was curious if statsmodels has an easy way of returning a coefficient $\epsilon$ that fits ...
844 views

### 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://...
172 views

### 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 <...
24 views

### Is a (beta)binomial model fitting for my response variable?

I am working on the evaluation of a speech recognition system we trained. The recognizer basically is given a query utterance containing a single word and should find images containing the appropriate ...
34 views

### Does pairwise correlation and multicollinearity matter in dispersion and zeroinflation model of glmmTMB?

I'm using glmmTMB to calculate beta-binomial GLMMs with nested and crossed random intercepts. I have overdispersed, zero-inflated data (assessed with Dharma). I use continuous terms in the very ...
419 views

### Capturing variability using ANOVA on date

I have a dataset of traffic count at several intersections at various dates. Most of the intersections were counted only once. I want to know if there is a significant daily and monthly variability in ...
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### Checking for Overdispersion in survey weighted data

I'm working with NHANES survey data. I'm trying to perform a survey weighted zero inflated Poisson model in R using a package called svyglm.zip. I want to check for Overdispersion. What statistical ...
451 views

### Sampling from under/over-dispersed count data in R

I am currently working a some datasets with count data in R, in which the response is the number of activities of a given type that were performed in one day by a population. For each type, I build ...
1k views

### Is an overdispersion parameter of 5.17 for GLMM with Beta family too high to yield reliable results?

I'm running a generalised linear mixed model with beta family on the effect of overhead cover (proportion ∈ (0,1)) on the proportion of birds scavenging from carrion left out in nature (proportion ∈ (...
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### Conf intervals on the dispersion coefficient of quasibinomial GLM or binomial GLMM with obs-level random effect

In this post I see how they calculate the confidence interval on the theta parameter of a negative binomial GLM: Confidence Interval for the Dispersion parameter of negbin distribution. I typically ...
166 views

### Modelling count data with extreme underdispersion - what distribution?

Suppose we have some count data, and we want to use a model that allows for "overdispersion" or "underdispersion" in the data (i.e., higher or lower variance than the Poisson distribution). Let $X$ ...
765 views

### GLMM: relationship between AIC, R squared and overdispersion?

First of all, I am a beginner in statistics, and I have been reading some questions and comments here regarding this matter but I am still a bit lost. My problem: I am constructing GLMM's in order to ...
101 views

### What is the difference between conditional and unconditional fixed effects?

What is the difference between unconditional and conditional (fixed effects negative binomial) regression models? A similar question was asked for quantile regression here: What is the difference ...
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### Analyze count data errors of commissions - is my model fine?

Is my model doing a good job here? I have errors of commission in a SART-task as a dependent variable and stress index and its interaction with a group (2 groups) as an independent variable. The ...
11k views

### Testing for overdispersion in logistic regression

R in Action (Kabacoff, 2011) suggests the following routine to test for overdispersion in a logistic regression: Fit logistic regression using binomial distribution: ...
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### Interpret the DHARMA simulation for the negative binomial regression

Our response variable is highly skewed and there is evidence of overdispersion as well. We used $pseudo R-squared$ and simulation using the DHARMA package to assess the quality of the model fit. How ...
128 views

### 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 ...
277 views

### Does clustering lead to overdispersion?

TL;DR Clustering is often cited as a source of overdispersion in count data. However, I seem to arrive at the conclusion that clustering actually reduces the dispersion. Could someone confirm this ...
36 views

### How to statistically test if the dispersion parameter in a negative binomial distribution is different?

I have a negative binomial distribution for two species. I have been able to fit (dummy) data to estimate mean (mu) and dispersion parameter (size). However, I want to statistically test if the two ...
50 views

### Is constructing a simulated model for a fixed SNR possible in binary regression?

When studying variable selection techniques like step-wise regression and the LASSO, a few studies employ a signal to ratio measure in order to control the amount of variability in a simulation. ...
160 views

### Comparing two heavily skewed, overdispersed counts

I have, as the title suggests, two heavily skewed, overdispersed histograms. The data ranges from 0 minutes to 85334 minutes. 90% of the data is below 15 minutes, and takes the form of a positive-...
29 views

### Test for equality of distributions for paired (overdispersed) count data

I have overdispersed count data with an excess of zeros. I split them into a training and a test set and tried to reconstruct its distribution through the training set. I then sampled from these ...
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### Is it better to overcome model singularity or overdispersion? … or to run a Bayesian glmer?

We are using a dataset extracted from a predation measurement experiment. My main question is: Do individuals catch more prey when they have tumors? We measure different control and tumoral ...
36 views

### How can I resolve overdispersion in a negative binomial model?

I started what seemed like a straightforward analysis, but I've gotten stuck with overdispersion in my negative binomial model. I would like to know which sites are different from each other in terms ...
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### Joint estimation of negative binomial dispersion parameter when measurements are independent per marker

Description of problem & experimental setup: In our experimental setup we have unique plasmids of which we measure the activity using RNAseq to get counts (generally follow negative binomial dist) ...
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### Overdispersion in logistic model

I'm relatively a newbie in R, and I've been trying to make a silly example of logistic regression to predict, according to Age and Sex whether someone dies of corona or not. I'm from Colombia, so my ...
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### Interepreting zeroinfl and glm.nb from my dataset

I am testing 4 predictors on overdispered count data with many zeros. The problem I am having is whether I have set-up my model correctly. my current model of ...
57 views

### How different from one does a dispersion ratio have to be to be considered significant?

I am in the process of conducting zero-inflated generalised mixed effects models with Poisson distributions and have been using the testDispersion() function of the DHARMa package in R to determine if ...
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### Error in a zero-inflated negative binomial model?

Following DHARMa diagnostic tests revealing zero-inflation (ratioObsSim = 32.663, p < 2.2e-16) and over-dispersion ...
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### Will usual tests state that data is overdispersed if response mean in poisson regression can vary greatly?

Assume I have 1000 responses from some count data, where each response follows the Poisson distribution with a mean (and variance) falling somewhere in the large range of 1 - 100. There is one ...
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### How come a Poisson GLMM predicts a higher overdispersion than in the observed data?

I am using package brms in R to fit a Bayesian generalized linear mixed model in which: the response variable is the count of a phenotypic structure (e.g., toes) ...
110 views

### What is the best way to deal with over-dispersion in a poisson GLMM?

I am currently in the process of trying to complete a poisson GLMM analysis with two fixed (with an interaction) and two random effects using the glmer() function of the lme4 package. Using the ...
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### Sum of non-identical Bernoulli is overdispersed or underdispersed Binomial?

Extra-binomial variation is defined in this Oxford Reference source: Greater variability in repeat estimates of a population proportion than would be expected if the population had a binomial ...
29 views

### How to perform negative binomial regression on gut microbiome data with over dispersion?

Thank you guys so much in advance! I got a table, containing the information of gut microbial composition of different groups, that is, the count data of different bacteria in each sample of different ...
116 views

### Interpreting results from a Poisson and Quasipoisson model

I used Poisson regression model to model how count of user actions on a website (dependent variable) are explained by website content (independent variables). The dependent variable distribution is ...
216 views

### 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 ...
15 views

### Trade-off between explaining variance and correcting overdispersion

I am fitting linear model. I happen to be working in R, and the specific model I'm fitting is a generalized additive model using the package mgcv, but I think all that is incidental to my question. ...
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### how to evaluate results with glmmPQL

First of all im thankfull for your attention. I have to evaluate the effect of 3 fixed effects in the vegetal coberture and i must use glmmPQL because my data has lineality condition problems and ...
59 views

### GLMER Overdispersion and Error messages

I have a data set which involves 30 binomial absence/presences totalled for a ratio out of 1, which is the total score of a test out of 30 marks. The data requires fitting one of my predictor ...
624 views

### Determining overdispersion of count variable in bayesian model (brms)

I am trying to determine whether my response count data are too overdispersed for a (brms) Bayesian poisson model. I constructed a poisson-generated response variable with low and high levels of noise/...
413 views

### Confused about over dispersion for my beta distribution

I have percentage data so I am using a beta distribution and I want to do a mixed-effect model so I am still trying to decide between glmmTMD or the brm packages. I saw somewhere that some ...
82 views

### How to choose the appropiate beta regression model type and variables?

Recently, I got my hands on modelling proportions [0,1]. Due to data type many of my variables are 0 and 1 inflated. Some of them are delicately affected by the bound values and some are heavily. I ...
3k views

### Types of dispersion parameter for binomial data

For a model with a binomial proportion as response variable, which is fitted with according to a binomial distribution, a dispersion parameter $\phi$ can be calculated, which is equal to the sum of ...
194 views

### Interpretation of zeroinfl vs. glm.nb results

I am trying to test effects of 3 predictors on overdispersed count data with many zeros, and a Vuong test suggested that a zero-inflated neg. binomial model would fit better than a negative binomial ...