[datatable-help] Data table syntax

[David Winsemius]

On Sep 5, 2010, at 11:38 AM, Damian Betebenner wrote:

> Thanks for the invaluable help on my previous questions. The speed  
> up in create summary tables has been immense and I’m enthused about  
> all the possibilities going forward.
>
> I’m currently stuck in trying to put together syntax for a “long”  
> for table. In the example below, each case is a unique Student by  
> Year combination. What I’m trying to do is take
> such a table, aggregate on the student’s  current year (i.e., 2009  
> in this data) SCHOOL_NUMBER, and calculate their mean score in the  
> previous year (i.e., 2008 in this data).
>
> If the file were “wide”, with each case representing a unique  
> student with separate variables for the year, then it would be easy  
> to break on the 2009 SCHOOL_NUMBER and take the
> mean of the 2008 SCORE.
>
> Is conversion of long to wide necessary to do this?

Probably not. Are you familiar with the "ave" function in base R?

>
>
>       STUDENT_ID SCHOOL_NUMBER YEAR SCORE
>  [1,]          1           100 2008    39
>  [2,]          1           200 2009    48
>  [3,]          2           100 2008    64
>  [4,]          2           200 2009    73
>  [5,]          3           100 2008    35
>  [6,]          3           200 2009    35
>  [7,]          4           100 2008    52
>  [8,]          4           200 2009    61
>  [9,]          5           100 2008    51
> [10,]          5           200 2009    58
> [11,]          6           300 2008    45
> [12,]          6           400 2009    55
> [13,]          7           300 2008    69
> [14,]          7           400 2009    77
> [15,]          8           300 2008    47
> [16,]          8           400 2009    47
> [17,]          9           300 2008    57
> [18,]          9           400 2009    58
> [19,]         10           300 2008    47
> [20,]         10           400 2009    53
>
>
> Thanks,
>
> Damian
>

David Winsemius, MD
West Hartford, CT