1. A new pharmaceutical entrant has been supplying fertility centers with a drug in competition with the three current products on the market. Merck Pharmaceuticals sells one of these products and wants to determine how big a threat this new entrant might be. It pulled market share data from its sales information system for a few months before and after the product was released and for fertility centers that adopted the new entrants product or not. The following table summarizes the information:

Market Condition	Pre-Entry	Post-Entry
Without Entry	37%	34%
With Entry	45%	33%

1. What is the time-series estimate of the effect of entry on Mercks market share from the markets with entry?
2. What is the cross-sectional estimate of the effect of entry on Mercks market share from the markets during the post-entry period?
3. What is the difference-in-difference estimate of the effect of entry on Mercks market share?
4. The Merck product was introduced over a decade ago but has been steadily losing ground to other products introduced 8 and 5 years ago. Would this affect the causal interpretation of the difference-in-difference estimator?
5. The newest product is more effective but has a worse risk profile for older mothers. Some centers specialize in treating older pregnant mothers. How does this affect the causal interpretation of the difference-in-difference estimator?