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Re: st: help to find maximum drop in a variable


From   smerryman@kc.rr.com
To   statalist@hsphsun2.harvard.edu
Subject   Re: st: help to find maximum drop in a variable
Date   Tue, 02 Mar 2004 08:31:02 -0600

Chris

How about this (it requires Nick Cox and Richard Goldstein's -spell- , use -findit spell- to locate and download):

cl, noobs
gen time = _n
tsset time
gen diff_pressure = d.pressure
gen sign = cond(diff>=0, 1 , -1)
spell sign
bysort _spell: gen cumul = sum(diff)
qui sum cumul
gen max_drop = cond(r(min)< 0, r(min), 0)
l, abb(16) noobs

Which produces the following: 

. cl, noobs

pressure
      90
     100
      90
      80
      70
      80
      65
      70
      90
      80
      70

. gen time = _n

. tsset time
        time variable:  time, 1 to 11

. gen diff_pressure = d.pressure
(1 missing value generated)

. gen sign = cond(diff>=0, 1 , -1)

. spell sign

. bysort _spell: gen cumul = sum(diff)

. qui sum cumul

. gen max_drop = cond(r(min)< 0, r(min), 0)

. l, abb(16) noobs

  +----------------------------------------------------------------------------------+
  | pressure   time   diff_pressure   sign   _spell   _seq   _end   cumul   max_drop |
  |----------------------------------------------------------------------------------|
  |       90      1               .      1        1      1      0       0        -30 |
  |      100      2              10      1        1      2      1      10        -30 |
  |       90      3             -10     -1        2      1      0     -10        -30 |
  |       80      4             -10     -1        2      2      0     -20        -30 |
  |       70      5             -10     -1        2      3      1     -30        -30 |
  |----------------------------------------------------------------------------------|
  |       80      6              10      1        3      1      1      10        -30 |
  |       65      7             -15     -1        4      1      1     -15        -30 |
  |       70      8               5      1        5      1      0       5        -30 |
  |       90      9              20      1        5      2      1      25        -30 |
  |       80     10             -10     -1        6      1      0     -10        -30 |
  |----------------------------------------------------------------------------------|
  |       70     11             -10     -1        6      2      1     -20        -30 |
  +----------------------------------------------------------------------------------+


Hope this helps,
Scott


----- Original Message ----- 
From: chris wallace <c.wallace@qmul.ac.uk> 
Date: Tuesday, March 2, 2004 6:59 am 
Subject: st: help to find maximum drop in a variable 

> My colleague has passed to me a query to which I can't find a 
> straightforward answer. I am hoping some of the experts in this group 
> could help? 
> 
> She has hourly barometric pressure data for many days. So the dataset 
> contains the variables 
> 
> year month day hour pressure 
> 
> (with 24 hourly observations per day). During any day the 
> pressure may 
> rise and fall. She wishes to generate a new variable containing the 
> maximum fall (constant within year-month-day groups). That is, 
> for a 
> fictitious series of pressure readings 
> 
> 90 100 90 80 70 80 65 70 90 80 70 
> 
> the maximum fall is 100-70=30. This is not simply the maximum - 
> minimum(100-65), since the pressure rises for one interval in that 
> period(70->80). 
> 
> She also makes the points: 
> - there may be more than one drop per day... i want the largest one 
> - if pressure has not dropped at all through out the day, the value 
> returned should be zero or negative 
> 
> Any suggestions? 
> 
> Many thanks, Chris. 
> 


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