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

[Rodrigo Refoios Camejo <refoioscamejo@gmail.com>]

Hi all,

I have just started fiddling around with STATA through reading the
manuals and having a look at the statalist discussions  whenever I
found a problem with my dataset.

I have 10 years 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 since the time they were launched. My idea is to fit a pricing
model that looks at variables related with 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.

My question is whether I should treat it as a time series.
Technically, although I will just include the L.price to control for
any price variation (I will probably use D2.price) and to pick up the
previous year leader’s market share, I believe I should do it.
However, how can I deal with the fact that some products were only
launched half-way the data timeframe, i.e. price==”0” & quantity==”0”
for all t before t launch. How do I do for STATA to select 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 # products in each
class in a quarter; and then assign to each product the # products in
each class at the time of that product’s launch?

I hope I was clear enough…many thanks in advance!

All the best,

Rodrigo

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