Re: st: generate the level series from the growth rate series

[annoporci <annoporci@gmail.com>]

> Evidently, levels can be reconstructed from changes given an initial  
> value:

This is very helpful. Thanks very much Nick.

I was about to send in my own best effort. It's redundant now of course.

I followed a roundabout way, so I'm thrilled to see how you did it shorter.

I had problems putting it into a foreach loop. Adapting your code may well  
solve those problems too (see Version 3 below).

Version 1 has no loop and works.
Version 2 has a loop and works.
Version 3 has a loop, uses tempname, works but is a real mess.


/* Creating a "Level" Index from the "Change" or "Rate of Change" */

 /// Without a loop
 clear all
 sysuse sp500
 gen growth = change[_n]/close[_n-1]
 gen close2 = .
 replace close2 = 1283.27 in 1
 gen t = _n
 tsset t
 replace close2 = close2[_n-1]*(1+growth[_n]) if t>1


 /// In loop without tempname
 clear all
 sysuse sp500
 sort date
 gen growth = change[_n]/close[_n-1]
 local ini = 1283.27
 local variables close
 foreach var of varlist `variables' {
   gen `var'2 = .
   replace `var'2 = `ini' in 1
   gen t = _n
   tsset t
   replace `var'2 = `var'2[_n-1]*(1+growth[_n]) if t>1
   drop t
 }
 ///


 /// In loop with tempname.
 /// It seemed like a good idea to use tempname, but probably wasn't!
 clear all
 sysuse sp500
 sort date
 gen growth = change[_n]/close[_n-1]
 local ini = 1283.27
 local variables close
 local t _n
 foreach var of varlist `variables' {
   tempname `var'2
   gen ``var'2' = .
   replace ``var'2' = `ini' in 1
   tempname tt
   gen `tt' = `t'
   tsset `tt'
   replace ``var'2' = ``var'2'[`tt'-1]*(1+growth[`tt']) if `tt'!=1
   gen `var'2 = ``var'2'
 }
///



Let me look into your code Nick, and learn from it.

:-)

---
Patrick Toche.


> . sysuse sp500
> . gen close2 = close[1] + sum(change)
> . assert close == close2
>
> If in your real data you have _percent_ changes (assuming equally
> spaced times and no missings or omissions):
>
> gen double level = 100
> replace level = level[_n-1] * (1 + pcchange / 100) if _n > 1
>
> This generalises to panels:
> bysort panelid (time) : replace level = level[_n-1] * (1 + pcchange /
> 100) if _n > 1
>
> Nick
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