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From | Rodrigo Refoios Camejo <refoioscamejo@gmail.com> |
To | statalist@hsphsun2.harvard.edu |
Subject | st: Time series - what to do before a product launch?; how to count n obs showing an attribute in each year? |
Date | Wed, 31 Mar 2010 16:47:47 +0100 |
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 * * 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/