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# Re: st: Relative Comparision between Observations

 From Robert Picard To statalist@hsphsun2.harvard.edu Subject Re: st: Relative Comparision between Observations Date Thu, 25 Aug 2011 15:54:53 -0400

```A simple and direct Stata solution to this problem is to use -cross-
to form all combinations of spreads and transactions and then drop
observations where the transaction date is not within the spread
dates. Obviously, this is not feasible given the size of the data in
question.

The following solution expands each spread by the number of calendar
days covered and creates a date variable that can be used to match
with the transaction dates. The -joinby- command is then used to form
all combinations of spread and transaction that occur on each calendar
day in the data.

You might have to further split the problem by month or year.

Hope this help,

Robert

*----------- begin example -------------
set seed 1234

clear
set obs 10000
gen sid = _n
gen start = mdy(1,1,2010) + int(runiform() * 365)
gen end = start + int(runiform() * 3)

* fake transaction data
clear
set obs 5000
gen tid = _n
gen tday = mdy(1,1,2010) + int(runiform() * 365)
sort tday
tempfile transactions
save "`transactions'"

* expand each spread by the number of days
gen ndays = end - start + 1
expand ndays

* each observation targets a specific day
sort sid
by sid: gen tday = start + _n - 1
keep sid ndays tday

* form all combinations of spreads and transactions
sort tday
joinby tday using "`transactions'"
format %d tday
sort sid tday tid

* If desired, reshape to wide
by sid: gen n = _n
sum n, meanonly
local nmax = r(max)
qui forvalues i = 1/`nmax' {
by sid: gen trans`i' = tid[`i']
}
by sid: keep if _n == 1
drop tday tid n

*------------ end example --------------

On Thu, Aug 25, 2011 at 10:56 AM, Jens Kruk <chaiselongue@gmx.de> wrote:
> one important note: the number of transactions per spread should be small, probably smaller than 10 for virtually every spread.
>
> Jens
>
>
> -------- Original-Nachricht --------
>> Datum: Thu, 25 Aug 2011 15:52:23 +0100
>> Von: Nick Cox <njcoxstata@gmail.com>
>> An: statalist@hsphsun2.harvard.edu
>> Betreff: Re: st: Relative Comparision between Observations
>
>> With that number, "my" approach can't get its shoes on, let alone run
>> down the track -- although it is I think what you asked for.
>>
>> I think you really need advice from people who do your kind of thing
>> in Stata, but unfortunately I am not one of them. I only have two
>> broad thoughts: think long not wide, and at some point -merge- will be
>>
>> Nick
>>
>> On Thu, Aug 25, 2011 at 3:43 PM,  <chaiselongue@gmx.de> wrote:
>> > Hi Nick,
>> > thanks a lot.
>> > The dataset contains 500 000 transactions (in addition to the 7 million
>> spreads), but I will use your approach as a starting point for an algorithm
>> that allows to cope with this large dataset.
>> >
>> > Any suggestion to get this done quickly is still very welcome.
>> >
>> >
>> > Best regards and thanks again,
>> >
>> > Jens
>> >
>> >
>> >
>> >
>> >
>> >
>> >
>> > -------- Original-Nachricht --------
>> >> Datum: Thu, 25 Aug 2011 15:20:58 +0100
>> >> Von: Nick Cox <njcoxstata@gmail.com>
>> >> An: statalist@hsphsun2.harvard.edu
>> >> Betreff: Re: st: Relative Comparision between Observations
>> >
>> >> For -transaction[2]- (e.g.) you can generate
>> >>
>> >> . gen within_2 = inrange(transaction[2], start, end) & isspread
>> >>
>> >> Is the number of transactions small enough to allow a variable for
>> >> every one of them?
>> >>
>> >> If so, this is crude but should work
>> >>
>> >> forval i = 1/`=_N' {
>> >>      if isspread[`i'] == 0 gen within_`i' =
>> inrange(transaction[`i'],
>> >> start, end) & isspread
>> >> }
>> >>
>> >> A visceral reaction is that getting the wrong data structure is
>> >> horribly easy here, but people who work with this kind of data may be
>> >> able to advise constructively.
>> >>
>> >> Nick
>> >>
>> >> On Thu, Aug 25, 2011 at 2:55 PM, Jens Kruk <chaiselongue@gmx.de> wrote:
>> >> > Hi Nick,
>> >> > lets say the data looks like this:
>> >> >
>> >> > 1_____1___________3________6______.
>> >> > 2_____0___________.________.______5
>> >> > 3_____1___________2________5______.
>> >> > 4_____0___________.________.______5.5
>> >> >
>> >> >
>> >> >
>> >> > now what I want Stata to do is to tell me (for example by creating
>> >> additional variables that contain the ids) that ids 2 and 4 occured
>> between
>> >> start and end date of observation 1 (5 and 5.5 are between 3 and 6) and
>> that id
>> >> 2 occured between the start and end date of spread 3 (5 is weakly
>> between
>> >> 2 and 5).
>> >> > A perfect result of the procedure would look like this:
>> >> >
>> >> > 1_____1___________3________6______.______________2_____4__
>> >> > 2_____0___________.________.______5______________._____.__
>> >> > 3_____1___________2________5______.______________2_____.__
>> >> > 4_____0___________.________.______5.5____________._____.__
>> >> >
>> >> >
>> >> > Best, Jens
>> >> >
>> >> >
>> >> >
>> >> >
>> >> > -------- Original-Nachricht --------
>> >> >> Datum: Thu, 25 Aug 2011 14:22:19 +0100
>> >> >> Von: Nick Cox <njcoxstata@gmail.com>
>> >> >> An: statalist@hsphsun2.harvard.edu
>> >> >> Betreff: Re: st: Relative Comparision between Observations
>> >> >
>> >> >> Please show a representative chunk of your data so that precisely
>> what
>> >> >>
>> >> >> Nick
>> >> >>
>> >> >> On Thu, Aug 25, 2011 at 2:09 PM,  <chaiselongue@gmx.de> wrote:
>> >> >>
>> >> >> > I want to perform the following task for a very large dataset (so
>> >> >> writing a Mata loop is probably not the solution): the dataset
>> consists
>> >> of two
>> >> >> sorts of data: spreads and transactions. Spreads do have a start and
>> an
>> >> end
>> >> >> date, while transactions only have a transaction date. Now I want to
>> >> know
>> >> >> whether some transaction happend between the start and end date of a
>> >> >> Ideally, I would like to have variables containing all the ids of
>> >> >> transactions that occured between the start and end data of the
>> >> for each
>> >> >> spread. Is there a way to use inexact matching or merging for this ?
>> >> >> > This should be a familiar problem, however, I do not have a clue
>> how
>> >> to
>> >> >> solve it.
>> >>
>> >> *
>> >> *   For searches and help try:
>> >> *   http://www.stata.com/help.cgi?search
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>> >> *   http://www.ats.ucla.edu/stat/stata/
>> >
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>> >
>>
>> *
>> *   For searches and help try:
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```