LongitudinalOverdispersedCounts {OpenMx} | R Documentation |

##
Longitudinal, Overdispersed Count Data

### Description

Four-timepoint longitudinal data generated from an arbitrary Monte Carlo simulation, for 1000 simulees. The response variable is a discrete count variable. There are three time-invariant covariates. The data are available in both "wide" and "long" format.

### Usage

data("LongitudinalOverdispersedCounts")

### Format

The "long" format dataframe, `longData`

, has 4000 rows and the following variables (columns):

`id`

: Factor; simulee ID code.

`tiem`

: Numeric; represents the time metric, wave of assessment.

`x1`

: Numeric; time-invariant covariate.

`x2`

: Numeric; time-invariant covariate.

`x3`

: Numeric; time-invariant covariate.

`y`

: Numeric; the response ("dependent") variable.

The "wide" format dataset, `wideData`

, is a numeric 1000x12 matrix containing the following variables (columns):

`id`

: Simulee ID code.

`x1`

: Time-invariant covariate.

`x3`

: Time-invariant covariate.

`x3`

: Time-invariant covariate.

`y0`

: Response at initial wave of assessment.

`y1`

: Response at first follow-up.

`y2`

: Response at second follow-up.

`y3`

: Response at third follow-up.

`t0`

: Time variable at initial wave of assessment (in this case, 0).

`t1`

: Time variable at first follow-up (in this case, 1).

`t2`

: Time variable at second follow-up (in this case, 2).

`t3`

: Time variable at third follow-up (in this case, 3).

### Examples

data(LongitudinalOverdispersedCounts)
head(wideData)
str(longData)
#Let's try ordinary least-squares (OLS) regression:
olsmod <- lm(y~tiem+x1+x2+x3, data=longData)
#We will see in the diagnostic plots that the residuals are poorly approximated by normality,
#and are heteroskedastic. We also know that the residuals are not independent of one another,
#because we have repeated-measures data:
plot(olsmod)
#In the summary, it looks like all of the regression coefficients are significantly different
#from zero, but we know that because the assumptions of OLS regression are violated that
#we should not trust its results:
summary(olsmod)
#Let's try a generalized linear model (GLM). We'll use the quasi-Poisson quasilikelihood
#function to see how well the y variable is approximated by a Poisson distribution
#(conditional on time and covariates):
glm.mod <- glm(y~tiem+x1+x2+x3, data=longData, family="quasipoisson")
#The estimate of the dispersion parameter should be about 1.0 if the data are
#conditionally Poisson. We can see that it is actually greater than 2,
#indicating overdispersion:
summary(glm.mod)

[Package

*OpenMx* version 2.8.3

Index]