Thesis (PhD)--Stellenbosch University, 2011. / ENGLISH ABSTRACT: Delineating the relevant product and geographic market is an important first step in competition inquiries, as it permits an assessment of market power and substitutability. Critics often argue that market definition is arbitrary and increasingly unnecessary, as modern econometric models can directly predict the competitive effects of a merger or anti-competitive practice. Yet practical constraints (such as limited data) and legal considerations (such as case law precedence) continue to support a formal definition of the relevant market. Within this context, this dissertation develops three tools to improve market definition: two empirical tools for cases with limited data and one conceptual decision-making tool to elucidate important factors and risks in market definition. The first tool for market definition involves a systematic analysis of consumer characteristics (i.e. the demographic and income profiles of consumers). Consumer characteristics can assist in defining markets as consumers with similar characteristics tend to switch to similar products following a price rise. Econometric models therefore incorporate consumer characteristics data to improve price elasticity estimates. Even though data constraints often prevent the use of econometric models, a systematic analysis of consumer characteristics can still be useful for market definition. Cluster analysis offers a statistical technique to group products on the basis of the similarity of their consumers. characteristics. A recently concluded partial radio station merger in South Africa offers a case study for the use of consumer characteristics in defining markets. The second tool, or set of tools, for defining markets involves using tests for price co-movement. Critics argue that price tests are not appropriate for defining markets, as these tests are based on the law of one price - which tests only for price linkages and not for the ability to raise prices. Price tests, however, are complements for existing market definition tools, rather than substitutes. Critics also argue that price tests suffer from low statistical power in discriminating close and less close substitutes. But these criticisms ignore inter alia the role of price tests as tools for gathering information and the range of price tests with better size and power properties that are available, including new stationarity tests and autoregressive models. A recently concluded investigation in the South African dairy industry offers price data to evaluate the market definition insights of various price tests. The third tool is conceptual in nature and involves a decision rule for defining markets. If market definition is a binary classification problem (a product is either 'in' or 'out' of the market), it faces risks of misclassification (incorrectly including or excluding a product). Analysts can manage these risks using a Bayesian decision rule that balances (1) the weight of evidence in favour of and against substitutability, (2) prior probabilities determined by previous cases and economic research, and (3) the loss function of the decision maker. The market definition approach adopted by the South African Competition Tribunal in the Primedia / Kaya FM merger investigation offers a useful case study to illustrate the implementation of such a rule in practice. / AFRIKAANSE OPSOMMING: Mededingingsake neem gewoonlik 'n aanvang met die afbakening van die relevante produk- en geografiese mark. Die markdefinisie-proses werp dikwels lig op markmag en substitusie-moontlikhede, en ondersteun dus die beoordeling van 'n mededingingsaak. Markdefinisie word egter deur kritici as arbitrer en selfs onnodig geag, veral aangesien ekonometriese modelle die uitwerking van 'n samesmelting of 'n teen-mededingende praktyk op mededinging direk kan voorspel. Tog verkies praktisyns steeds om markte formeel af te baken op grond van sowel praktiese oorwegings (insluitend databeperkings wat ekonometriese modellering bemoeilik) as regsoorwegings (insluitend die rol van presedentereg). Hierdie proefskrif ontwikkel dus drie hulpmiddels vir die definisie van markte: twee empiriese hulpmiddels vir gevalle waar data beperk is sowel as 'n denkhulpmiddel om o.a. risiko's rondom markdefinisie te bestuur. Die eerste hulpmiddel vir die definisie van markte behels die sistematiese analise van verbruikerseienskappe, insluitend die demografiese en inkomste-profiel van verbruikers. Verbruikerseienskappe werp lig op substitusie, aangesien soortgelyke verbruikers neig om na soortgelyke produkte te verwissel na aanleiding van 'n prysstyging. Ekonometriese modelle maak derhalwe van data omtrent verbruikerseienskappe gebruik om beramings van pryselastisiteit te verbeter. Hoewel databeperkings dikwels ekonometriese modellering beperk, kan verbruikerseienskappe op sigself steeds nuttig wees vir die afbakening van die mark. Trosanalise bied 'n statistiese metode vir 'n stelselmatige ondersoek van verbruikerseienskappe vir markdefinisie, deurdat dit produkte op grond van gelyksoortige verbruikerseienskappe groepeer. 'n Onlangse ondersoek in Suid-Afrika rakende die gedeeltelike samesmelting van Primedia and Kaya FM radiostasies bied data om die gebruik van trosanalise en verbruikerseienskappe vir markdefinisie-doeleindes te illustreer. Die tweede hulpmiddel vir markdefinisie behels statistiese toetse vir verwantskappe tussen prystydreekse van verskillende produkte of streke. Hierdie prystoetse is gebaseer op die wet van een prys en beklemtoon prysverwantskappe eerder as die vermoë om pryse te verhoog (wat die uiteindelike fokus in mededingingsbeleid is). Hierdie klem verminder egter nie noodwendig die insigte wat prystoetse bied nie, aangesien markdefinisie dikwels 'n omvattende analise verg. Prystoetse se statistiese onderskeidingsvermoe word ook dikwels deur kritici as swak beskryf. Hierdie tegniese kritiek beskou prystoetse as eng-gedefinieerde hipotesetoetse eerder as hulpmiddels vir die verkenning van substitusiepatrone. Voorts ignoreer hierdie tegniese kritiek 'n verskeidenheid nuwe prystoetse met beter onderskeidingsvermoë, insluitend nuwe toetse vir stasioneriteit en nuwe autoregressiewe modelle. 'n Onlangse mededingingsondersoek in die Suid-Afrikaanse melkindustrie verskaf prysdata om die verrigting van verskillende prystoetse vir geografiese markdefinisie te ondersoek. Die derde hulpmiddel vir die definisie van markte behels 'n besluitnemingsreël. Hiervolgens word markdefinisie as 'n binêre klassifikasieprobleem beskou, waar 'n produk of streek 'binne' of 'buite' die mark geplaas moet word. Gegewe dat hierdie klassifikasie onder toestande van onsekerheid geskied, is markdefinisie blootgestel aan risiko's van wanklassifikasie. Praktisyns kan hierdie risiko‟s bestuur deur gebruik te maak van 'n Bayesiaanse besluitnemingsreël. Sodanige reël balanseer (1) die gewig van getuienis ten gunste van en teen substitusie, (2) a priori waarskynlikhede soos bepaal deur vorige mededingingsake en akademiese navorsing, en (3) die verliesfunksie van die besluitnemer. Die benadering van die Suid-Afrikaanse Mededingingstribunaal in die saak rakende die gedeeltelike samesmelting van Primedia en Kaya FM bied 'n nuttige gevallestudie om hierdie beginsels te demonstreer.