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

[Christophe Genolini]

Thanks for yours answers. So if I understand:
 - Trajectories are continuous, the other are discrete.
 - The difference between time series and longitudinal is that time 
series are made at regular time whereas longitudinal are not ?
 - Repeated measures are over a short period of time.


So if I measure the  weight of my patient daily during one week, it will 
be repeated measure ; if I measure it once a week during one year,
it will time series ; if I measure it once a week during one year but 
with some "missing week", it will longitudinal data ? 

Well I guess it is not as simple at that, but is it the idea ?

Christophe


> 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.
>
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> Robert A. LaBudde, PhD, PAS, Dpl. ACAFS  e-mail: ral at lcfltd.com
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