Effective Access Time

The Effective Access Time is the average time it takes to access something in memory: ?

$$\begin{array}{rl}EAT& =HR\times \text{cache access}+MR\times (\text{cache access}+\text{main memory access})\\ & =\text{cache access}+MR\times (\text{main memory access})\end{array}$$

Our aim is to reduce the effective time as much as possible.