  Market definition is an important component in the premerger investigation,
but the models used in the market definition have not developed much in the
past three decades since the Critical Loss Analysis (CLA) was proposed in 1989.
The CLA helps the Hypothetical Monopolist Test to determine whether the
hypothetical monopolist is going to profit from the small but significant and
non-transitory increase in price (SSNIP). However, the CLA has long been
criticized by academic scholars for its tendency to conclude a narrow market.
Although the CLA was adopted by the 2010 Horizontal Merger Guidelines (the 2010
Guidelines), the criticisms are likely still valid. In this dissertation, we
discussed the mathematical deduction of CLA, the data used, and the SSNIP
defined by the Agencies. Based on our research, we concluded that the narrow
market conclusion was due to the incorrect implementation of the CLA; not the
model itself. On the other hand, there are other unresolvable problems in the
CLA and the Hypothetical Monopolist Test. The SSNIP test and the CLA are bright
resolutions for market definition problem during their time, but we have more
advanced tools to solve the task nowadays. In this dissertation, we propose a
model which is based directly on the multi-dimensional substitutability between
the products and is capable of maximizing the substitutability of product
features within each group. Since the 2010 Guidelines does not exclude the use
of models other than the ones mentioned by the Guidelines, our method can
hopefully supplement the current models to show a better picture of the
substitutive relations and provide a more stable definition of the market.
