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st: Re: Portfolio returns per month, drop return duplicates


From   "M@rk " <[email protected]>
To   <[email protected]>
Subject   st: Re: Portfolio returns per month, drop return duplicates
Date   Tue, 26 Nov 2013 11:14:10 +0100

Thanks for the replies! @ Nick: I now have the data that I need to proceed. Its probably not the way to delete duplicates, but it worked for me. @Sergiy: Collapse works, but I have multiple portfolios. Not only PRB1, but als PRB2, PRB3 etcetera (120 portfolios). It uses mean values, which is no problem as the return figures in a month are the same for each observation. Can I combine multiple portfolios (/variables) when using this command? Or is it only applicable for 1 variable?

Kind Regards,

Mark Krap

-----Oorspronkelijk bericht----- From: Sergiy Radyakin
Sent: Monday, November 25, 2013 5:52 PM
To: [email protected]
Subject: Re: st: Portfolio returns per month, drop return duplicates

On Mon, Nov 25, 2013 at 7:27 AM, M@rk <[email protected]> wrote:
Dear Users,

For my research I need to have portfolio returns for each month from 1926
till 2012. I already calculated the weighted returns for each portfolio and
summed them up (by date) by creating a  new variable (PRB1). The only
problem is that this sum is not given once for each month, but is given for
each stock observation in a month. So if the total of the weighted returns
is, let's say, 0.026 in March 2006 its mentioned for each stock observation
in March 2006. I tried to downsize this by using the duplicates drop [in]
command, but I get the error message 'invalid obs no'. Does anyone know how
I can get the portfolio returns once per month, as for my research I only

Besides what Nick has already suggested:

#1
sort year month
by year month: keep if _n==1

#2
sort year month
collapse PRB1, by(year month)

note that you need year unless your month is cmc.


In the command that you quoted "duplicates drop PRB1" you are relying
on a random fact that your portfolio returns would vary uniquely by
time, which is commonly true, but not necessary (e.g. in the period of
stability you could have returns for e.g. 3 months being same).You
should seek duplicates in terms of time, not values!

Best, Sergiy

need portfolio returns per month from 1926 till 2012 in order to do the
regressions. I am using Stata 12 SE (Windows 7 64-bit).

I look forward to the reactions.


Kind Regards,

Mark Krap

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