Notice: On April 23, 2014, Statalist moved from an email list to a forum, based at statalist.org.

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

From |
Nick Cox <njcoxstata@gmail.com> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Re: Stack trick by Nicholas Cox |

Date |
Mon, 4 Mar 2013 09:50:33 +0000 |

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. * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/faqs/resources/statalist-faq/ * http://www.ats.ucla.edu/stat/stata/

**Follow-Ups**:**Re: st: Re: Stack trick by Nicholas Cox***From:*Michael Stewart <michaelstewartresearch@gmail.com>

**Re: st: Re: Stack trick by Nicholas Cox***From:*Nick Cox <njcoxstata@gmail.com>

**References**:**st: Re: Stack trick by Nicholas Cox***From:*Michael Stewart <michaelstewartresearch@gmail.com>

- Prev by Date:
**Re: st: Independent variable censoring** - Next by Date:
**Re: st: data transformation** - Previous by thread:
**st: Re: Stack trick by Nicholas Cox** - Next by thread:
**Re: st: Re: Stack trick by Nicholas Cox** - Index(es):