## Is a mixed/ random effects model required if fixed effects model shows no pattern in the residuals?

I’ve got some data on discrete flood events (response variable – duration) on several rivers.

I’ve modelled the response variable by site and time using a Generalized Linear Model:

```
mod = glm(duration ~ site + time)
```

I’ve then looked at the residual plots and cannot find any evidence for temporal correlation in the residuals. Does this mean that this model approach is valid (and that using a mixed/ random effects model is unnecessary) (or should one always use mixed/ random effects models for time-series data)?

Any links to reading references regarding this subject would be brilliant,