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I'm trying to see how boldness of individuals change over time. For this, I constructed a repeated measures random intercept random slope model with boldness scores (measured as latency to resume movement) as the continuous response variable, Trial (time) as fixed effects and Trial|ID as random effects. I used the rstanarm package for this, as lmer resulted in singular fits. Model diagnostics revealed that the model fit was okay. I then used sjPlot package to obtain the random intercept random slope plot (without confidence intervals). Below is the plot.

enter image description here

Each line in the plot represents an individual. As is evident from the figure, some lines are not straight, but rather show slight curvature. According to my knowledge, each line is the line of best fit for individual boldness scores over time. So how can a line of best fit show curvature? Is there something wrong with the model? Examining the model diagnostics showed that the rhat values were close to 1, no divergent transition of iterations and no autocorrelation. I'm also attaching the model fit below.

enter image description here

Any help will be much appreciated. Thanks much in advance!

Edit: Model formula is: mod<-stan_lmer(Latency.s.~Trial+(Trial|uniqueID),data=mydata). Trial is an integer from 1 to 4. uniqueID is a factor with 201 levels. The response variable Latency.s. appears multimodal with a left skew, but after Box-Cox transformation still appears multimodal with less skew (0.3). Below are the histograms of the non-transformed and transformed response variable.

Histogram of Response variable (Latency) Non-transformed response variable

Box-Cox transformed response variable Box-Cox Transformed response variable

Adding the summary of the data: Latency.s.: Min=12, max=1114, median=156.5,mean=255.17, 1st quartile=77.75, 3rd quartile=383.25. Trial: min=1, max=4, median=2.5, mean= 2.48, 1st quartile=1.75, 3rd quartile=3.0.

The random slopes plot look similar to the plot attached above even after running the model with the Box-Cox transformed response variable. Reconfirming: variable Trial is definitely not a factor, but an integer.

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  • $\begingroup$ Can we have more information please? What is your model formula? What is summary() of your data set; in particular, is Trial numeric or categorical (factor)? $\endgroup$ – Ben Bolker Dec 23 '18 at 18:28
  • $\begingroup$ Model formula is: mod<-stan_lmer(Latency.s.~Trial+(Trial|uniqueID),data=mydata). Trial is integer from 1 to 4. uniqueID is a factor with 201 levels. The response variable Latency.s. appears multimodal with a left skew, bu after Box-Cox transformation still appears multimodal with less skew (0.3). Adding the summary of the data: Latency.s. - Min=12, max=1114, median=156.5,mean=255.17, 1st quartile=77.75, 3rd quartile=383.25. Trial - min=1, max=4, median=2.5, mean= 2.48, 1st quartile=1.75, 3rd quartile=3.0. $\endgroup$ – BP86 Dec 23 '18 at 18:51
  • $\begingroup$ Thanks. Can you edit this information into your question please? And can you confirm that Trial is definitely not a factor? $\endgroup$ – Ben Bolker Dec 23 '18 at 18:51
  • $\begingroup$ The random slopes plot looks similar to the plot attached above, even after running the model with the box-cox transformed response. $\endgroup$ – BP86 Dec 23 '18 at 18:52

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