My table is laid out as such:

ID (int) What (varchar 20) TimeStamp (smalldatetime)
------- ------------- ---------------
73 Start <T1>
73 Misc <T2>
73 End <T3>
81 Start <T1'>
81 Misc <T2'>
81 End <T3'>
....

I need to calculate End - Start for each unique ID (i.e. T3-T1 and
T3'-T1') and then take the average of those (2 in this case) entries.

Any help is appreciated.

Alex.