[datatable-help] Data table syntax

[Damian Betebenner]

Hi David,

Thanks for the quick and thoughtful reply.

Sorry for not being clearer.

There are two schools that the students attended in 2009 (200 and 400). I'd like to break on those, and calculate the mean for all the students in those two schools but for their 2008 scores.

Thus, the output would have 2 rows:

YEAR, 2009_SCHOOL_NUMBER, 2008_SCORE_MEAN

2009  200    54.4
2009  400    53

Thanks for considering this,

Damian


Damian Betebenner
Center for Assessment
PO Box 351
Dover, NH   03821-0351
 
Phone (office): (603) 516-7900
Phone (cell): (857) 234-2474
Fax: (603) 516-7910

dbetebenner at nciea.org
www.nciea.org




-----Original Message-----
From: David Winsemius [mailto:dwinsemius at comcast.net] 
Sent: Sunday, September 05, 2010 1:03 PM
To: David Winsemius
Cc: Damian Betebenner; datatable-help at lists.r-forge.r-project.org
Subject: Re: [datatable-help] Data table syntax


On Sep 5, 2010, at 12:43 PM, David Winsemius wrote:

>
> 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.

But there is only one 2008 SCORE for each student???

>>
>> Is conversion of long to wide necessary to do this?
>
> Probably not. Are you familiar with the "ave" function in base R?

I am having some difficulty understanding the structure of the desired  
output. I initially thought it might be something like:
  rd.txt <-
function(txt, header=TRUE, ...) {
      rd <- read.table(textConnection(txt), header=header, ...)
        closeAllConnections()
      rd }
txt <- rd.txt("STUDENT_ID SCHOOL_NUMBER YEAR SCORE
          1           100 2008    39
          1           200 2009    48
          2           100 2008    64
          2           200 2009    73
          3           100 2008    35
          3           200 2009    35
          4           100 2008    52
          4           200 2009    61
          5           100 2008    51
          5           200 2009    58
          6           300 2008    45
          6           400 2009    55
          7           300 2008    69
          7           400 2009    77
          8           300 2008    47
          8           400 2009    47
          9           300 2008    57
          9           400 2009    58
         10           300 2008    47
         10           400 2009    53")
dtxt <- data.table(txt)

 > dtxt$avScr <- dtxt[ , ave(SCORE, list(STUDENT_ID))]  # returns a  
vector as long as its input
 > dtxt

But now I am wondering if you wanted:

 > dtxt[ , tapply(SCORE, list(STUDENT_ID), mean)]  # returns vector  
only as long as product of category levels.
    1    2    3    4    5    6    7    8    9   10
43.5 68.5 35.0 56.5 54.5 50.0 73.0 47.0 57.5 50.0

>
>>
>>
>>      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