20 Overdispersion
Overdispersion describes the observation that variation is higher than would be expected.
20.1 Recognising (and testing for) overdispersion
library(lme4)
data(grouseticks)
summary(grouseticks)## INDEX TICKS BROOD HEIGHT YEAR
## 1 : 1 Min. : 0.00 606 : 10 Min. :403.0 95:117
## 2 : 1 1st Qu.: 0.00 602 : 9 1st Qu.:430.0 96:155
## 3 : 1 Median : 2.00 537 : 7 Median :457.0 97:131
## 4 : 1 Mean : 6.37 601 : 7 Mean :462.2
## 5 : 1 3rd Qu.: 6.00 643 : 7 3rd Qu.:494.0
## 6 : 1 Max. :85.00 711 : 7 Max. :533.0
## (Other):397 (Other):356
## LOCATION cHEIGHT
## 14 : 24 Min. :-59.241
## 4 : 20 1st Qu.:-32.241
## 19 : 20 Median : -5.241
## 28 : 19 Mean : 0.000
## 50 : 17 3rd Qu.: 31.759
## 36 : 16 Max. : 70.759
## (Other):287