[R] the first and last observation for each subject

[Carlos J. Gil Bellosta]

Hello,

First, order your data by ID and time.

The columns you want in your output dataframe are then

unique(ID),

tapply( x, ID, function( z ) z[ 1 ] )

and

tapply( y, ID, function( z ) z[ lenght( z ) ] - z[ 1 ] )

Best regards,

Carlos J. Gil Bellosta
http://www.datanalytics.com


On Fri, 2009-01-02 at 17:20 +0800, gallon li wrote:
> I have the following data
> 
> ID x y time
> 1  10 20 0
> 1  10 30 1
> 1 10 40 2
> 2 12 23 0
> 2 12 25 1
> 2 12 28 2
> 2 12 38 3
> 3 5 10 0
> 3 5 15 2
> .....
> 
> x is time invariant, ID is the subject id number, y is changing over time.
> 
> I want to find out the difference between the first and last observed y
> value for each subject and get a table like
> 
> ID x y
> 1 10 20
> 2 12 15
> 3 5 5
> ......
> 
> Is there any easy way to generate the data set?
> 
> 	[[alternative HTML version deleted]]
> 
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