Interpreting Oracle Statistics

http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/instance_tune.htm#i16848

Gather statistics that cover the time when the instance had the performance problem. If you previously captured baseline data for comparison, then you can compare the current data to the data from the baseline that most represents the problem workload.
When comparing two reports, ensure that the two reports are from times where the system was running comparable workloads.

Using Wait Event Statistics to Drill Down to Bottlenecks

Whenever an Oracle process waits for something, it records the wait using one of a set of predefined wait events. These wait events are grouped in wait classes. The Idle wait class groups all events that a process waits for when it does not have work to do and is waiting for more work to perform. Non-idle events indicate nonproductive time spent waiting for a resource or action to complete.
The most effective way to use wait event data is to order the events by the wait time. This is only possible if TIMED_STATISTICS is set to true. Otherwise, the wait events can only be ranked by the number of times waited, which is often not the ordering that best represents the problem.
To get an indication of where time is spent, follow these steps:

1.      Examine the data collection for V$SYSTEM_EVENT. The events of interest should be ranked by wait time.

Identify the wait events that have the most significant percentage of wait time. To determine the percentage of wait time, add the total wait time for all wait events, excluding idle events, such as Null event, SQL*Net message from client, SQL*Net message to client, and SQL*Net more data to client. Calculate the relative percentage of the five most prominent events by dividing each event's wait time by the total time waited for all events.

Alternatively, look at the Top 5 Timed Events section at the beginning of the Automatic Workload Repository report. This section automatically orders the wait events (omitting idle events), and calculates the relative percentage:

Top 5 Timed Events

~~~~~~~~~~~~~~~~~~                                                                    

                                                           % Total

Event                                Waits    Time (s) Call Time

----------------------------- ------------ ----------- ---------

CPU time                                           559     88.80

log file parallel write              2,181          28      4.42

SQL*Net more data from client      516,611          27      4.24

db file parallel write              13,383          13      2.04

db file sequential read                563           2       .27

In some situations, there might be a few events with similar percentages. This can provide extra evidence if all the events are related to the same type of resource request (for example, all I/O related events).

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A table of common wait events is listed in below link
http://download.oracle.com/docs/cd/B19306_01/server.102/b14211/instance_tune.htm#g44034