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From |
"Nick Cox" <n.j.cox@durham.ac.uk> |

To |
<statalist@hsphsun2.harvard.edu> |

Subject |
RE: st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year? |

Date |
Tue, 13 Apr 2010 14:28:37 +0100 |

One technique here is simply tag and sum. 1. Tag what you want to count with 1 and anything else with 0. 2. Sum the 1s and 0s. Evidently the result is the count. Thus if the question were: how many distinct products which are generic in every subclass and every quarter? egen tag = tag(product subclass quarter) if generic == 1 egen count = total(tag), by(subclass quarter) should do the trick. There's much more discussion within various FAQs and articles: . search distinct Keyword search Keywords: distinct Search: (1) Official help files, FAQs, Examples, SJs, and STBs Search of official help files, FAQs, Examples, SJs, and STBs [P] levelsof . . . . . . . . . . . . . . . . . . . . . Levels of variable (help levelsof) FAQ . . . . . . . . . . . . . . Calculating the number of distinct values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox 9/06 How do I calculate the number of distinct values seen so far? http://www.stata.com/support/faqs/data/distinctvalues.html FAQ . . . . . . . . . Counting distinct strings across a set of variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox 7/04 How do I count the number of distinct strings across a set of variables? http://www.stata.com/support/faqs/data/distinctstrings.html FAQ . . . . . . . . . . . . . . . . . . . Number of distinct observations . . . . . . . . . . . . . . . . . . . . . . . N. J. Cox and G. Longton 4/02 How do I compute the number of distinct observations? http://www.stata.com/support/faqs/data/distinct.html SJ-9-1 pr0046 . . . . . . . . . . . . . . . . . . . Speaking Stata: Rowwise (help rowsort, rowranks if installed) . . . . . . . . . . . N. J. Cox Q1/09 SJ 9(1):137--157 shows how to exploit functions, egen functions, and Mata for working rowwise; rowsort and rowranks are introduced SJ-8-4 dm0042 . . . . . . . . . . . . Speaking Stata: Distinct observations (help distinct if installed) . . . . . . N. J. Cox and G. M. Longton Q4/08 SJ 8(4):557--568 shows how to answer questions about distinct observations from first principles; provides a convenience command Nick n.j.cox@durham.ac.uk Rodrigo Refoios Camejo (As expected) you were right and it was coming back wrong because I had not dropped the obs corresponding to the products before launch. But I realised have another problem now... I want to count the number of products that are generic in a subclass for a given quarter, but because it's retrieving the number of presentation (id) that are generic in a subclass for a given quarter. This because in each product there are several presentation each one with one obs per quarter. How can I count the # products in each subclass in a given quarter that are generic instead of the # presentations (id)? The dataset looks something like this: id product subclass generic quarter 1 1 1 1 1 2 1 1 1 2 3 1 1 1 3 4 2 1 0 1 5 2 1 0 2 6 2 1 0 3 7 3 2 1 1 8 3 2 1 2 9 3 2 1 3 On Mon, Apr 12, 2010 at 8:45 PM, Nick Cox <n.j.cox@durham.ac.uk> wrote: > I don't think so. The -by(subclass quarter)- subdivides observations > according to combinations of the two variables, which I thought was what > you wanted. Rodrigo Refoios Camejo > Thanks for your quick reply, Nick... however, your suggestion is > returning only the generic==1 in each subclass and ignoring the by > quarter, i.e. sum is the same for all obs of thin each subclass > irrespectively of the quarter it was observed at. > > Any idea what may be going wrong? I've grouped quarter, I've sorted by > quarter subclass and still the same result... On 4/12/10, Nick Cox <n.j.cox@durham.ac.uk> wrote: >> The larger question here is "How should I model this?" which is >> difficult at the best of times and in any case better left to >> subject-matter experts. >> >> The smaller question is more my thing. >> >> egen sum = total(generic == 1), by(subclass quarter) >> >> may be the sort of solution you need. Rodrigo Refoios Camejo [edited] >> I have 10 years of panel data on pricing and quantities sold of >> pharmaceuticals. For each presentation (i.e. dosage and package) of >> each product I have data on the prices and quantities sold in each >> quarter. My idea is to fit a regression model with price as the >> dependent variable and independent variables related to competition >> like: # products in the therapeutic class at time of launch; # >> generics in the therapeutic class at time t; market share of market >> leader at time t; price of market leader at time t-1; etc. >> >> How can I deal with the fact that some products were only launched >> half-way the data timeframe and some were discontinued after launch, >> i.e. when price=="0" & quantity=="0" for all t before t launch and for >> some after t launch. How can I have Stata including in the regression >> only the data for which price and quantity is available? Simply drop >> if price==0? Will it not treat it as missing if I do so? >> >> Another side question, how can I make Stata count the # products with >> a particular characteristic (e.g. generic==1) marketed in each group >> (subclass) of drugs in each quarter? And then assign to each product >> the count corresponding to the quarter in which that product was >> launched? * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**Re: st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year?***From:*Rodrigo Refoios Camejo <refoioscamejo@gmail.com>

**RE: st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year?***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**Re: st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year?***From:*Rodrigo Refoios Camejo <refoioscamejo@gmail.com>

**RE: st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year?***From:*"Nick Cox" <n.j.cox@durham.ac.uk>

**Re: st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year?***From:*Rodrigo Refoios Camejo <refoioscamejo@gmail.com>

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