st: Re: Lagged price

["Brad Wright" <bradwright@unc.edu>]

I don't know of a formula per se, but you should definitely take advantage 
of Stata's time-series operators for lagging variables.

You must first use tsset or xtset to tell Stata what format your data are 
in, and then creating lags is very simply done by using "L.varname" for 1 
lag or "2L.varname" for 2 periods and so on.

Then you can write a piece of code that will automate this for you so that 
you can loop through all the combinations, stopping when you find the first 
"non-matching" period.

-Brad

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----- Original Message ----- 
From: "Beatrice Crozza" <beatrice.crozza@gmail.com>
To: "statalist" <statalist@hsphsun2.harvard.edu>
Sent: Friday, January 28, 2011 5:08 PM
Subject: st: Lagged price


> Dear All,
>
> I want to infer the trade direction with a tick test, comparing two
> consecutive prices.
> I have a problem with Stata in order to construct the lagged price.
> When two consecutive prices are the same, I should go back one more
> lag, however, often I need to compare my price with a price of three
> or more previous periods.
>
> Here there is an example of my dataset:
>
> date          time         price
> 03jan2000 93157 148.25
> 03jan2000 93200 148.25
> 03jan2000 93201 148.27
> 03jan2000 93202 148.25
> 03jan2000 93203 148.25
> 03jan2000 93208 148.25
> 03jan2000 93211 148.25
> 03jan2000 93212 148.25
> 03jan2000 93215 148.15625
> 03jan2000 93225 148.25
>
> if I compare price at 93212 with the previous one, it is the same and
> also if I compare it with the price at 93208, so I should go further
> until I reach the price of 93201 which is different.
>
> If I want to do the difference for two consecutive prices I can write:
> price[_n]-price[n-1]
> a lag further:
> price[_n]-price[_n-2]
> and so on
>
> However, I would like to know if there is a formula to do the
> difference between to prices, until I find a price which is different
> from the one in consideration.
>
> Thank you very much or your help.
>
> Bea
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