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# Re: st: RE: Calculate variances of subsamples

 From Lars Knuth To statalist@hsphsun2.harvard.edu Subject Re: st: RE: Calculate variances of subsamples Date Sat, 5 Jun 2010 21:15:59 +0200

```Dear Statalisters,

I want to share the results, maybe there is someone (in the future),
who has the same problem to solve. The input came almost completely
from Martin.
The program takes price data for a large number of stocks, calculates
returns, calculates then variances for every stock individually many
times using a rolling window. The variances are in the right order to
be compared in the cross-section.

This is also the beginning of my next and last problem:
I have all the variances. I now need to compare at every point in time
all the variances for the different stocks (Exxon, Microsoft etc),
rank them with the lowest variance first (at every point in time),
then build 10 portfolios (it will be more than 1000 stocks) where the
first portfolio for example exists of the stocks with the 10% lowest
variances, the second includes the next ten percent etc.

For those portfolios, the return has to be calculated (so the datasets
have to be merged again). Then it can be tested (t-test) whether the
low variances stock portfolio has a statistically significantly lower
return than the high variance portfolio.

As I said, if someone (Martin?) has an idea for that, I would be more
than thankful since in this case I can finish my Stata work for the
moment.

clear*
set more off

*create resultsfile
cap erase resultsfile.dta
di in red _rc

clear*
gen start=.
gen end=.
gen _stat_1=.
gen str15 kindofreturn=""

save resultsfile, replace

use "\3105.dta", clear
gen int time=_n

*Return calculation
foreach price in pricemsci priceex pricemsft priceapp pricege pricepg
pricejnj pricebp {
gen double `price'ret=`price'[_n]/`price'[_n-1]-1 if _n>1
}
renpfix price

*loop to get -rolling- results for each stock
foreach ret in exret msciret msftret appret geret pgret jnjret bpret{
preserve
tsset time
rolling r(Var), window(60) clear: su `ret'
gen kindofreturn="`ret'"
append using resultsfile
save resultsfile, replace
restore
}

u resultsfile, clear
ren _stat_1 Variance
sort kindofreturn start
l, sepby(kindofreturn) noo

bys kindofreturn: gen int time=_n
reshape wide Variance, i(time) j(kindofreturn) string
renpfix Variance
list, noo

2010/6/5 Martin Weiss <martin.weiss1@gmx.de>:
>
> <>
>
> ***********
> clear*
>
> input Var str16 stock
> 0.00234    exxon
> .05654    exxon
> 0.13444    exxon
> 0.99388    microsoft
> .4342     microsoft
> 0.42445    microsoft
> 0.42444    intel
> 0.32443       intel
> 0.23434     intel
> end
>
> bys stock: gen int time=_n
> reshape wide Var, i(time) j(stock) string
> renpfix Var
> list, noo
> ***********
>
>
> HTH
> Martin
>
>
> -----Original Message-----
> From: owner-statalist@hsphsun2.harvard.edu
> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Lars Knuth
> Sent: Samstag, 5. Juni 2010 17:49
> To: statalist@hsphsun2.harvard.edu
> Subject: Re: st: RE: Calculate variances of subsamples
>
> Oh, my explanation was probably irritating. It was just for
> illustration. I have two columns, one with the numbers, the other
> having the strings. What I need to have in a new file is just the
> numbers.
>
> 0.00234(exxon) says that there should be the 0.00234, which depends to
> exxon etc. It were of course nice if the new variable including the
> exxon numbers would be named exxon, the one having the numbers for
> microsoft could be named microsoft etc. However, the important part
> concerns just the numbers.
>
> 2010/6/5 Martin Weiss <martin.weiss1@gmx.de>:
>>
>> <>
>>
>> Your problem may turn out to be easily solved with a -reshape-. It is not
> a
>> good idea, though, to have "0.00234(exxon)" in a cell of your data, as
> this
>> would have to be stored as a string, precluding any further processing of
>> the number. Did you write it as an illustration, or do you really want the
>> cell to contain the string?
>>
>>
>> HTH
>> Martin
>>
>>
>> -----Original Message-----
>> From: owner-statalist@hsphsun2.harvard.edu
>> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Lars Knuth
>> Sent: Samstag, 5. Juni 2010 17:28
>> To: statalist@hsphsun2.harvard.edu
>> Subject: Re: st: RE: Calculate variances of subsamples
>>
>> Ok, great, it took some time, but I finally understood Martin`s
>> code... this is a great way of learning more about STATA.
>> My next problem is that I have a variable with the variances for 536
>> -rolling- steps for each of the stocks.
>> It looks like this:
>> Variance         stock name
>> 0.00234          exxon
>> ...........          exxon
>> 0.13444          exxon
>> 0.99388          microsoft
>> ...........           microsoft
>> 0.42445          microsoft
>> 0.42444          intel
>> ........              intel
>> 0.23434           intel
>>
>> What I would like to have is the following:
>>
>> 0.00234(exxon)          0.99388(microsoft)     0.42444(intel)
>> ...........                    ............
> ............
>> 0.13444                    0.42445                     0.23434(intel)
>>
>> I could do gen varexxon=Variance if stockname=="exxon"
>> and that for all the stocks. But even if I do so I get variables with
>> a lot of missings and I can not write the variances horizontally next
>> to each other.
>>
>> But they are from the same time (because of -rolling-) and I need them
>> to be horizontally ordered without the missings.
>>
>> I hope my problem becomes clear. I guess what I miss is just a small
>> command.
>> Thank you in advance for any hint!
>>
>> 2010/6/2 Martin Weiss <martin.weiss1@gmx.de>:
>>>
>>> <>
>>>
>>> You could of course issue the -rolling- call with -clear- present, -save-
>>> the result to a new file and reload your "3105.dta" to start anew for the
>>> next stock. The datasets thus -saved- could be -append-ed to form one big
>>> dataset afterwards. -postfile- is also an option, as always.
>>>
>>> BTW, you may be better of with the lag operator "L." for your return
>>> calculations.
>>>
>>>
>>> HTH
>>> Martin
>>>
>>> -----Original Message-----
>>> From: owner-statalist@hsphsun2.harvard.edu
>>> [mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Lars Knuth
>>> Sent: Mittwoch, 2. Juni 2010 20:22
>>> To: statalist
>>> Subject: st: Calculate variances of subsamples
>>>
>>> Dear listers,
>>>
>>> I have to say thanks to Martin, the recommendation of rolling was
>>> great. Unfortunately, I have now a few problems with the
>>> implementation.
>>> 1. -rolling- works with the "clear" option, but without it does not
>>> ("rolling r(Var), window(60) clear: summarize exret" works)
>>> 2. I need the data to calculate and store the variances for more than
>>> 1000 stock price returns in the end, so can I somehow keep all the
>>> data and then perform -rolling- in a loop?
>>> 3. Is there also an opportunity to perform the return calculation in a
>> loop?
>>>
>>> I am attaching parts of the code I have so far. Any ideas would be of
>>> great help to me.
>>>
>>> clear*
>>> use "C:\...\3105.dta", clear
>>>
>>> gen int time=_n
>>> * Return calculation
>>> gen double exret=ex[_n]/ex[_n-1]-1 if _n>1
>>> gen double msciret=msci[_n]/msci[_n-1]-1 if _n>1
>>> gen double msftret=msft[_n]/msft[_n-1]-1 if _n>1
>>> gen double appret=app[_n]/app[_n-1]-1 if _n>1
>>> gen double geret=ge[_n]/ge[_n-1]-1 if _n>1
>>> gen double pgret=pg[_n]/pg[_n-1]-1 if _n>1
>>> gen double jnjret=jnj[_n]/jnj[_n-1]-1 if _n>1
>>> gen double bpret=bp[_n]/bp[_n-1]-1 if _n>1
>>>
>>> tsset time
>>>
>>> * Rolling
>>> rolling r(Var), window(60): summarize exret
>>> rolling r(Var), window(60): summarize msciret
>>> rolling r(Var), window(60): summarize msftret
>>> *
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>>
>> *
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>
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```