In this section, the relevant studies in the literature are summarized. According to Table 1, there is volatility in stock returns. This volatility shows the existence of asymmetric information and the leverage effect in stock returns. There are limited numbers of studies on the permanence of a shock affecting the system and the results are mixed. On the other hand, no information could be found in the literature about the explanatory power of the estimation models. The main restrictions for all volatility models are the knowledge and skill of the researcher.