[R-sig-ME] Help with nested and crossed effects in models with 2- and 3-way interactions

[Mollie Brooks]

I’m guessing that the problem with mod3 could be another instance of confusion with nested effects. 

As originally written, the random effects in mod3 are
(1|DayOfWeek/date) = (1|DayOfWeek) + (1| DayOfWeek:date)
The second term doesn’t make sense to me when each date can only be accompanied by either 0 or 1 for DayOfWeek.

Maybe you want (1|Subject) + (1|Date) in mod3. That model could address both hypotheses.

cheers,
Mollie

> On 23Aug 2020, at 12:50, Michal Kahn <michalkahn10 using gmail.com> wrote:
> 
> Hello there! I am running a mixed model in lmer, testing the effects of
> Covid restrictions on sleep, comparing 2 cohorts of individuals- one from
> 2019 and one from 2020, coded 0/1 (between subjects). Each individual was
> measured repeatedly for ~130 consecutive nights, and each row in the
> dataset represents a single night. I also have a binary Lockdown IV, where
> each night is coded 0/1 to indicate if it was before/after restrictions
> were imposed in 2020 (and the equivalent dates for 2019). Finally, I have a
> DayOfWeek IV, where each night is coded 0/1 to indicate if it represents a
> weekday/weekend night. The simplified dataset looks something like:
> 
> [image: enter image description here] <https://i.stack.imgur.com/Ouuhw.png>
> 
> My hypotheses are: (1) there will be a Cohort by Lockdown interaction
> effect on sleep; and (2) there will be a Cohort by Lockdown by DayOfWeek
> interaction effect on sleep.
> 
> For hypothesis 1, I ran:
> 
> mod1<- lmer(sleep ~ Cohort*Lockdown + (1|Subject) + (1|Date), data = COVID,
> REML=FALSE)
> 
> Results seem reasonable, but I think I am not accounting for random slopes.
> I have tried to model the slopes as follows, but the model failed to
> converge.
> 
> mod2<- lmer(sleep ~ Cohort*Lockdown + (Lockdown|Subject), data = COVID,
> REML=FALSE)
> 
> As for the 2nd hypothesis, if I understand correctly, nights are nested
> within DayOfWeek, which are crossed with Lockdown (since each level of
> Lockdown includes both weekdays and weekends). I tried the following code,
> but am getting a singular fit warning (boundary (singular) fit: see
> ?isSingular)
> 
> mod3<- lmer(sleep ~ Cohort * Lockdown * DayOfWeek + (1|DayOfWeek/date),
> data = COVID, REML=FALSE)
> 
> Could anyone direct me as to what should be changed in these models? Many
> thanks in advance for your help!
> Mika
> 
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