1. The PLE (Page Life Expectancy) in the report is the minimum value in the reporting period. So PLE could drop to zero somewhere in certain periods.
2. Average Response Time is calculated by averaging the execution time of the Response Time collection over the time period. The execution time of the collection measures the round trip time for the SQL to be sent to the SQL Server instance, executed there and the result returned to the Diagnostic Server.
3. Average IOs per second is calculated by averaging the sum of the performance counters 'page reads/sec' and 'page writes/sec' read from the sys.dm_os_performance_counters DMV.