Re: st: question: how do i...

[Phil Schumm <pschumm@uchicago.edu>]

At 1:53 AM +0000 3/7/05, israel solaresbravo wrote:
> i want to construct a variable called margin of victory:
>
> my data set looks something like this:
>
>
> state	votesA	votesB	votesC	votesD
> 1	10	1	6	21
> 2	11	2	7	22
> 3	12	3	8	13
> 4	1	4	2	3
> 5	2	5	3	7
>
>
> my question is:
>
> how do i create a variable called margin_of_victory = votes for 1st 
> place - votes for 2nd place?, i.e. 21-10 for state 1, 22 - 11 for 
> state 2, 13 - 12 for state 3, 4 - 3 for state 4, etc.

If you reshape your data from wide to long form, then this is a snap. 
Here are your data:

. li

     +-------------------------------------------+
     | state   votesa   votesb   votesc   votesd |
     |-------------------------------------------|
  1. |     1       10        1        6       21 |
  2. |     2       11        2        7       22 |
  3. |     3       12        3        8       13 |
  4. |     4        1        4        2        3 |
  5. |     5        2        5        3        7 |
     +-------------------------------------------+

Now, try the following (output omitted):

. reshape long votes, i(state) j(candidate) string
. bys state (votes): gen margin = votes[_N] - votes[_N-1]
. reshape wide

which produces exactly what you want:

. li

     +----------------------------------------------------+
     | state   votesa   votesb   votesc   votesd   margin |
     |----------------------------------------------------|
  1. |     1       10        1        6       21       11 |
  2. |     2       11        2        7       22       11 |
  3. |     3       12        3        8       13        1 |
  4. |     4        1        4        2        3        1 |
  5. |     5        2        5        3        7        2 |
     +----------------------------------------------------+

As you can see, the trick is in using -reshape-.  If you're not 
familiar with what -reshape- does, use -list- after the first reshape 
and take a look, then read [R] reshape.  Once the data are in long 
form, we sort by state and number of votes, and then within each 
state, compute the difference between the largest number of votes and 
the next largest (hint: _N refers to the total number of observations 
within the state, and votes[_N] is the last observation within the 
state (which, since the observations are sorted by number of votes, 
is the observation with the largest vote count)).


-- Phil
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