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I am confused how to analyze this dataset in R: There are 2 islands, 2 sites were selected (nonrandom) per each island, 6 transects randomly were located on each site, then the number of corals was counted in three categories: bleached, partial-bleached and health. The data are collected for 2 years.

enter image description here

The transect (random effect) is nested within sites, and sites (fixed effect) within island (fixed effect), but I have counted on categorical data. I am confused which analyses should I use to compare the spatial and temporal variation in each category of health condition. I think generalized linear mixed model is perfect but multivariate abundance model may be the best. Which test (analysis) in which package is the best fit to my data? How should I deal with the categories, is it fixed effect factor?

Any suggestion would be appreciated.


I ran the model as:

model <- glmer(No.coral ~ island + health_status + (1|island:site:transect), data = data, family=poisson)
summary(model)

Generalized linear mixed model fit by maximum likelihood (Laplace Approximation) ['glmerMod']
     Family: poisson  ( log )
    Formula: No.coral ~ island + health_status + (1 | island:site:transect)
       Data: data
    
         AIC      BIC   logLik deviance df.resid 
     33065.5  33100.5 -16527.8  33055.5     8059 
    
    Scaled residuals: 
        Min      1Q  Median      3Q     Max 
     -1.733  -0.961  -0.567  -0.284 117.509 

Random effects:
 Groups               Name        Variance Std.Dev.
 island:site:transect (Intercept) 0.1081   0.3289  
Number of obs: 8064, groups:  island:site:transect, 32

Fixed effects:
               Estimate Std. Error z value Pr(>|z|)    
(Intercept)    -2.99554    0.10110  -29.63   <2e-16 ***
islandL         1.38643    0.12018   11.54   <2e-16 ***
health_statusM  2.29915    0.05599   41.06   <2e-16 ***
health_statusS  2.31649    0.05595   41.40   <2e-16 ***
---
Signif. codes:  0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Correlation of Fixed Effects:
            (Intr) islndL hlth_M
islandL     -0.618              
helth_sttsM -0.503  0.000       
helth_sttsS -0.503  0.000  0.909

My questions are: Is my model written correctly? why the model output is dismissed 'health_status_H'? Also, I don't know how to show the variation (increasing/decreasing) of each health status during the 4 years of observation (not only show the significance of variations as above)? (before, I told that the samplings were conducted during 2 years, by mistake). I also need comparisons ONLY among years at each island and each site. Many thanks.

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I don't know what a multivariate abundance model is, but as you thought, you should be able to use a generalised linear mixed model here. 6 is a rather low number for random intercepts, so I would also fit a non-mixed model, and fit Transect as a fixed effect, and compare the inferences from both. Hopefully they will be consistent.

Since you have count data, a good place to start is with a poisson model. If it turns out that you have excess zeros, then you can use a zero inflated model poisson model, and if you over or under dispersion, you can use a negative binomial instead of a poisson.

If you are using R then I would suggest starting with the mixed_model function in the GLMMadaptive package or the glmer function in the lme4 package.

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  • $\begingroup$ Dear Robert Long Thanks for your answer. So, in the GLMM how should I introduce the column of 'health_status' to the model? Is it fixed or random factor? Also, the year, here, is covariate, right? $\endgroup$
    – fgh
    Feb 19 at 19:18
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    $\begingroup$ You're welcome :) health_status and year will both be fixed effects (covariates) $\endgroup$ Feb 19 at 19:38
  • $\begingroup$ Due to character limitation here, I add my comment as an answer. I would appreciate any help you can provide. $\endgroup$
    – fgh
    Feb 21 at 13:53
  • $\begingroup$ Please don't post an answer to add more info. You can edit your question by using the Edit facility to add more info. Anyway, (1|island:site:transect) does not make sense. It should be just (1 | transect). health_status_H is included in the intercept. For the time effect you need to include year as a fixed effect, as I said in my last comment. $\endgroup$ Feb 21 at 16:01
  • $\begingroup$ Finally, I fit the model as: 'modelF8 <- glmmTMB(No.coral ~ (1|transect) + year +island + site + health_status + genus, data = data, family = nbinom1)' . Anova shows the significant differences between sites, however, I need to show the difference of sites in term of islands, i.e. nested design (sites within islands) has to be included. I don't know how to include nested design in glmmtmb. Also, I think there is some interaction such as 'health_status*genus'. But including such interaction leads to error of non-positive-definite Hessian matrix. $\endgroup$
    – fgh
    Mar 11 at 9:54

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