[R] "time series", "longitudinal data" or "trajectories"

[Robert A LaBudde]

At 04:02 PM 6/5/2009, Christophe Genolini wrote:
>Hi the list
>
>Strictly speaking, this is not a "R" question, but I need the 
>information for the
>creation of a package. My question is about vocabulary: What is the 
>difference between
>"time series", "longitudinal data" and "trajectories"?
>
>Sincerely
>
>Christophe

"Longitudinal" data are measurements over long periods of time, often 
at irregular periods, but consistent across subjects.

"Repeated measures" data are replicates at the same point in time, or 
over a short period of time (e.g., laboratory experiments).

"Time series" typically have constant increments of time and 
typically a stochastic character, although this term might be 
considered all-encompassing for all measurements at different times.

"Trajectory" implies a continuous curve in time, as opposed to 
discrete times. "Trajectory" also implies an underlying causal model, 
as it is a term from kinematics.

I hope this helps.

================================================================
Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
Least Cost Formulations, Ltd.            URL: http://lcfltd.com/
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