# Can a factor be regarded as both random and fixed effect?

I have a question about nested mixed effect model. For example I have species A with different populations; these populations belong to two kinds of habitat types (with or without predators). So I have population nested within habitat type, and I measured the body size of species A. I want to know if habitat type have a significant effect on body size or not. Thus, I got one model like this:

m1<-lmer(body_size~habitat_type+(1|year)+(1|habitat_type/population))
m2<-lmer(body_size~habitat_type+(1|year)+(1|habitat_type)+(1|habitat_type:population))


From http://conjugateprior.org/2013/01/formulae-in-r-anova/, I think m1 equals m2. But I was wodering why we regard habitat_type as both fixed effect and random effect? Does this make sense? Or I should just use the following model (remove habitat_type as random effect):

m3<-lmer(body_size~habitat_type+(1|year)+(1|habitat_type:population))


outputs for models:

   > summary(m2)
Linear mixed model fit by REML ['lmerMod']
Formula: body_size~ habitat_type + (1 | year) + (1 | habitat_type/population)
Data: exuv

REML criterion at convergence: 4219.3

Scaled residuals:
Min      1Q  Median      3Q     Max
-3.5214 -0.6482  0.0054  0.6369  3.2242

Random effects:
Groups                  Name        Variance Std.Dev.
population:habitat_type (Intercept) 0.20850  0.4566
year                    (Intercept) 0.06942  0.2635
habitat_type            (Intercept) 0.08963  0.2994
Residual                            0.73031  0.8546
Number of obs: 1625, groups:
population:habitat_type, 50; year, 19; habitat_type, 2

Fixed effects:
Estimate Std. Error t value
(Intercept)       0.3916     0.3275   1.196
habitat_typeinv  -0.6325     0.4509  -1.403

Correlation of Fixed Effects:
(Intr)
habtt_typnv -0.699

> summary(m3)
Linear mixed model fit by REML ['lmerMod']
Formula: body_size~ habitat_type + (1 | year) + (1 | habitat_type:population)
Data: exuv

REML criterion at convergence: 4219.3

Scaled residuals:
Min      1Q  Median      3Q     Max
-3.5214 -0.6482  0.0054  0.6369  3.2242

Random effects:
Groups                  Name        Variance Std.Dev.
habitat_type:population (Intercept) 0.20850  0.4566
year                    (Intercept) 0.06942  0.2635
Residual                            0.73031  0.8546
Number of obs: 1625, groups:  habitat_type:population, 50; year, 19

Fixed effects:
Estimate Std. Error t value
(Intercept)       0.3916     0.1329   2.947
habitat_typeinv  -0.6325     0.1549  -4.082

Correlation of Fixed Effects:
(Intr)
habtt_typnv -0.661

> summary(m4)
Linear mixed model fit by REML ['lmerMod']
Formula: body_size~ habitat_type + (1 | year) + (1 | population)
Data: exuv

REML criterion at convergence: 4219.3

Scaled residuals:
Min      1Q  Median      3Q     Max
-3.5214 -0.6482  0.0054  0.6369  3.2242

Random effects:
Groups     Name        Variance Std.Dev.
population (Intercept) 0.20850  0.4566
year       (Intercept) 0.06942  0.2635
Residual               0.73031  0.8546
Number of obs: 1625, groups:  population, 50; year, 19

Fixed effects:
Estimate Std. Error t value
(Intercept)       0.3916     0.1329   2.947
habitat_typeinv  -0.6325     0.1549  -4.082

Correlation of Fixed Effects:
(Intr)
habtt_typnv -0.661

> summary(m2ml)
Linear mixed model fit by maximum likelihood  ['lmerMod']
Formula: body_size~ habitat_type + (1 | year) + (1 | habitat_type/population)
Data: exuv

AIC      BIC   logLik deviance df.resid
4226.6   4259.0  -2107.3   4214.6     1619

Scaled residuals:
Min      1Q  Median      3Q     Max
-3.5215 -0.6493  0.0047  0.6360  3.2245

Random effects:
Groups                  Name        Variance  Std.Dev.
population:habitat_type (Intercept) 1.993e-01 4.464e-01
year                    (Intercept) 6.363e-02 2.523e-01
habitat_type            (Intercept) 4.365e-18 2.089e-09
Residual                            7.305e-01 8.547e-01
Number of obs: 1625, groups:
population:habitat_type, 50; year, 19; habitat_type, 2

Fixed effects:
Estimate Std. Error t value
(Intercept)       0.3889     0.1297   3.000
habitat_typeinv  -0.6299     0.1519  -4.147

Correlation of Fixed Effects:
(Intr)
habtt_typnv -0.664

• I found some answers with similar questions. It is very helpful. See stats.stackexchange.com/questions/70556/…. – Bin Apr 12 '17 at 12:53
• So what is the output when you fit these models? If m2 estimates zero variance for (1|habitat_type), then it's equal to m3. That's what I would expect (because you are right: if there is fixed term for habitat_type then there is nothing left for the random term to capture). – amoeba says Reinstate Monica Apr 12 '17 at 15:09
• You @amoeba got a good point. I tried with m2, m3 and another new model m4 (with random factor (1|population), m4<-lmer(body_size~habitat_type+(1|population))). I found they almost generated the same output, especially between m3 and m4. In m2 model, habitat_type as random effect still has 0.08963 (variance) and 0.2994 (std. dev.). Thus, in my situation, I think I can use either (1|population) or (1|habitat_type:population), because the results are exactly the same. – Bin Apr 18 '17 at 17:00
• How about using maximum likelihood, i.e., setting REML = FALSE for lmer? Can you provide the complete output summary of all the models you tried? – Randel Apr 18 '17 at 21:45
• Thank you very much @JiebiaoWang. I followed your advice and use the setting REML=F. Then for m2 the variance=4.3e-18, Std.Dev.=2.1e-09. Almost zero @amoeba. This is amazing. But this further makes me confused about how to choose between REML and ML. I checked some information about REML and ML differences. In my mind, usually REML is preferred. – Bin Apr 19 '17 at 13:16