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st: Dates, strings, formats & labels: tab / tabstat / table


From   "Allan Reese (Cefas)" <allan.reese@cefas.co.uk>
To   <statalist@hsphsun2.harvard.edu>
Subject   st: Dates, strings, formats & labels: tab / tabstat / table
Date   Mon, 13 Sep 2010 12:30:18 +0100

This is the type of behaviour that puts people off computers and causes
them to think folk who promote Stata are nerds - maybe I am for thinking
it worth writing up ;-)

The objective: summarize a data table as monthly statistics - in time
order.  Usually the data comes as a spreadsheet with the date as a
string. I always recommend writing the month as a name to avoid
ambiguity of day and month.

Today's detonator: having converted the "month year" string to a date,
the following commands gave irritatingly inconsistent interpretations.  

* Here's the input string variable, which sorts alphabetically
. tab month

      month |      Freq.     Percent        Cum.
------------+-----------------------------------
     APR 10 |         38        6.92        6.92
     AUG 09 |         37        6.74       13.66
     DEC 09 |         38        6.92       20.58
     FEB 10 |         38        6.92       27.50
     JAN 10 |         37        6.74       34.24
     JUL 09 |         38        6.92       41.17
     JUL 10 |         37        6.74       47.91
     JUN 09 |         37        6.74       54.64
     JUN 10 |         34        6.19       60.84
     MAR 10 |         39        7.10       67.94
     MAY 09 |         32        5.83       73.77
     MAY 10 |         38        6.92       80.69
     NOV 09 |         30        5.46       86.16
     OCT 09 |         37        6.74       92.90
     SEP 09 |         39        7.10      100.00
------------+-----------------------------------
      Total |        549      100.00

* -gen- a date and format it %tdMY

. desc date

              storage  display     value
variable name   type   format      label      variable label
------------------------------------------------------------------------
-----------------------------------------------
date            float  %tdMY                  

* -tab- insists it was the 1st of each month
. tab date

       date |      Freq.     Percent        Cum.
------------+-----------------------------------
  01may2009 |         32        5.83        5.83
  01jun2009 |         37        6.74       12.57
  01jul2009 |         38        6.92       19.49
  01aug2009 |         37        6.74       26.23
  01sep2009 |         39        7.10       33.33
  01oct2009 |         37        6.74       40.07
  01nov2009 |         30        5.46       45.54
  01dec2009 |         38        6.92       52.46
  01jan2010 |         37        6.74       59.20
  01feb2010 |         38        6.92       66.12
  01mar2010 |         39        7.10       73.22
  01apr2010 |         38        6.92       80.15
  01may2010 |         38        6.92       87.07
  01jun2010 |         34        6.19       93.26
  01jul2010 |         37        6.74      100.00
------------+-----------------------------------
      Total |        549      100.00

* though -table- can handle the concept ...
. table date

------------------------
       date |      Freq.
------------+-----------
      May09 |         32
     June09 |         37
     July09 |         38
   August09 |         37
September09 |         39
  October09 |         37
 November09 |         30
 December09 |         38
  January10 |         37
 February10 |         38
    March10 |         39
    April10 |         38
      May10 |         38
     June10 |         34
     July10 |         37
------------------------ 

* but -table- will handle only five statistics at a time, and I wanted
six.  So tried -tabstat- and got a shock
. tabstat row row ,  stat(mean) by(date)

Summary statistics: mean
  by categories of: date 

     date |       row       row
----------+--------------------
    18018 |      16.5      16.5
    18049 |        54        54
    18079 |      94.5      94.5
    18110 |       135       135
    18141 |       176       176
    18171 |       217       217
    18202 |     253.5     253.5
    18232 |     290.5     290.5
    18263 |       331       331
    18294 |     371.5     371.5
    18322 |       413       413
    18353 |     454.5     454.5
    18383 |     494.5     494.5
    18414 |     532.5     532.5
    18444 |       570       570
----------+--------------------
    Total |  295.7687  295.7687
-------------------------------

* Why have I asked for the same variable in two columns?  Because when I
simplified the table to send the report ...
. tabstat row ,  stat(mean) by(date)

Summary for variables: row
     by categories of: date 

     date |      mean
----------+----------
      May09 |      16.5
     June09 |        54
     July09 |      94.5
   August09 |       135
September09 |       176
  October09 |       217
 November09 |     253.5
 December09 |     290.5
  January10 |       331
 February10 |     371.5
    March10 |       413
    April10 |     454.5
      May10 |     494.5
     June10 |     532.5
     July10 |       570
----------+----------
    Total |  295.7687
---------------------

With one statistic the -by- was output formatted, but I could not get
the header and total lines to align even using -labelwidth- and
-longstub-.

Having found -tabstat- deformatted the stub, another approach was to
-egen- the group(date) values to give monthindex numbers, but they need
the month-year labels.  The only way to generate labels from an existing
variable appears to be -encode-, but that sorts the values
alphabetically.  

I typed in the labels -label define month 1 "May 09" etc- and attached
those to the index.

Like the bank says, "There must be a better way".


R Allan Reese
Senior statistician, Cefas
The Nothe, Weymouth DT4 8UB 

Tel: +44 (0)1305 206614 -direct
Fax: +44 (0)1305 206601 

www.cefas.co.uk 




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