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# 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" To Subject RE: st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year? Date Mon, 12 Apr 2010 18:57:15 +0100

```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.

Nick
n.j.cox@durham.ac.uk

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?

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