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Re: st: Re: Stack trick by Nicholas Cox


From   Michael Stewart <michaelstewartresearch@gmail.com>
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: Re: Stack trick by Nicholas Cox
Date   Mon, 4 Mar 2013 15:10:34 -0500

Thanks a lot for clarification.
I dont have specific task in mind but I was just commenting  on  the
utility of the set of commands written by you for the stack trick
Thanks again
Mike

On Mon, Mar 4, 2013 at 2:49 PM, Nick Cox <njcoxstata@gmail.com> wrote:
> Thanks, not quite sure which specific task(s) you have in mind. But consider
>
> sysuse auto
> logit foreign mpg
> regplot, jitter(1)
>
> predict predicted
> stripplot mpg, over(foreign) stack height(0.2)  addplot(mspline
> predicted mpg, bands(100))
>
> where -regplot- is from SJ and -stripplot- is from SSC.
>
> Nick
>
> On Mon, Mar 4, 2013 at 7:33 PM, Michael Stewart
> <michaelstewartresearch@gmail.com> wrote:
>> Thanks a lot for such a nice explanation and taking time to type explain it.
>> I understand it now.
>> These are such a nice  set of commands & are extremely useful, I hope
>> one of the experienced stata programmers out there could write a user
>> written program for it .!!
>> Thanks a lot Nick
>> Mike
>>
>>
>> On Mon, Mar 4, 2013 at 4:50 AM, Nick Cox <njcoxstata@gmail.com> wrote:
>>> The reference is to a Speaking Stata column in the _Stata Journal_.
>>> This is accessible to all  in
>>>
>>> http://stata-journal.com/sjpdf.html?articlenum=gr0004
>>>
>>> The example uses Stata's auto data and can be replicated by
>>>
>>> sysuse auto
>>>
>>> bysort foreign mpg: gen foreign2 = ///
>>> cond(foreign==1,  1-0.1*(_n-1)/7, foreign+0.1* (_n-1)/7)
>>>
>>> The results can be inspected. Here is enough to give a flavour of what
>>> is produced
>>>
>>> . l foreign mpg foreign2
>>>
>>>      +---------------------------+
>>>      |  foreign   mpg   foreign2 |
>>>      |---------------------------|
>>>   1. | Domestic    12          0 |
>>>   2. | Domestic    12   .0142857 |
>>>   3. | Domestic    14          0 |
>>>   4. | Domestic    14   .0142857 |
>>>   5. | Domestic    14   .0285714 |
>>>      |---------------------------|
>>>   6. | Domestic    14   .0428571 |
>>>   7. | Domestic    14   .0571429 |
>>>   8. | Domestic    15          0 |
>>>   9. | Domestic    15   .0142857 |
>>>
>>> <snip>
>>>
>>>  53. |  Foreign    14          1 |
>>>  54. |  Foreign    17          1 |
>>>  55. |  Foreign    17   .9857143 |
>>>      |---------------------------|
>>>  56. |  Foreign    18          1 |
>>>  57. |  Foreign    18   .9857143 |
>>>  58. |  Foreign    21          1 |
>>>  59. |  Foreign    21   .9857143 |
>>>  60. |  Foreign    23          1 |
>>>      |---------------------------|
>>>  61. |  Foreign    23   .9857143 |
>>>  62. |  Foreign    23   .9714286 |
>>>  63. |  Foreign    24          1 |
>>>  64. |  Foreign    25          1 |
>>>  65. |  Foreign    25   .9857143 |
>>>      |---------------------------|
>>>  66. |  Foreign    25   .9714286 |
>>>  67. |  Foreign    25   .9571428 |
>>>  68. |  Foreign    26          1 |
>>>  69. |  Foreign    28          1 |
>>>  70. |  Foreign    30          1 |
>>>      |---------------------------|
>>>  71. |  Foreign    31          1 |
>>>  72. |  Foreign    35          1 |
>>>  73. |  Foreign    35   .9857143 |
>>>  74. |  Foreign    41          1 |
>>>      +---------------------------+
>>>
>>> The idea is to get a y coordinate at which to plot each pair of values
>>> in a scatter plot of -foreign- versus -mpg-. The context is that we
>>> are plotting a logit fit from -logit foreign mpg- and we are adding
>>> the raw data at the top and bottom of the plot as what are now often
>>> called as rugs.
>>>
>>> Consider the last observation, which is the only observation with
>>> foreign = 1 (Foreign), mpg = 41. We can just plot it as y = 1, x = 41.
>>>
>>> The previous two observations tie at foreign = 1, mpg = 35. If we
>>> plotted them, the marker symbols would just be superimposed.
>>>
>>> So we stack them vertically. One can be plotted at y = 1, x = 35, but
>>> the other must be nudged downwards from y = 1.
>>>
>>> A similar decision applies for values with foreign = 0. Pairs that
>>> occur once only can be plotted at y = 0, x = mpg value, but ties must
>>> be separated to be discernible.
>>>
>>> The general rule for this dataset -- chosen after experiment -- was
>>>
>>> cond(foreign==1,  1-0.1*(_n-1)/7, foreign+0.1* (_n-1)/7)
>>>
>>> meaning
>>>
>>> for foreign = 1, use y = 1 if _n == 1, 1 - 0.1/7 if _n == 2, and so on.
>>>
>>> for foreign = 0, use y = 0 if _n == 1, 0 + 0.1/7 if _n == 3, and so on.
>>>
>>> The -cond()- function handles both cases at once. -search cond, sj-
>>> for access to a 2005 tutorial by David Kantor and myself if needed.
>>>
>>> What is _n here? It is crucial that the observation number _n is
>>> counted _within_ distinct groups of -foreign mpg-. -search by, sj- for
>>> access to a 2002 tutorial if needed.
>>>
>>> 7 is just a choice that works well in this dataset, or so I thought.
>>>
>>> There is no use of options in this code.
>>>
>>> Nick
>>>
>>> On Sun, Mar 3, 2013 at 8:10 PM, Michael Stewart
>>> <michaelstewartresearch@gmail.com> wrote:
>>>
>>>> I am a novice and trying to learn stata graphics.I read Speaking Stat
>>>> by Nick, vol 4 , number 2, page 190-215 regarding Graphing Categorical
>>>> and compositional data.
>>>> Nick writes a conditional statement on page 193.I could not understand
>>>> the second option
>>>> His cond statement is bysort foreign mpg:gen foreign2=cond(foreign==1,
>>>> 1-0.1*(_n-1)/7, foreign+0.1*(_n-1)/7)
>>>> I cannot understand  what does 1-0.1*(_n-1)/7 and
>>>> foreign+0.1*(_n-1)/7  compute and their purpose.I tried to read the
>>>> article but am still at loss.
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>>
>>
>>
>> --
>> Thank you ,
>> Yours Sincerely,
>> Mike.
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-- 
Thank you ,
Yours Sincerely,
Mike.
*
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