Group aggregate on logs

for example, i have the following 7 logs,

2011-DEC-17 slotid="Location-Maps-US-Sunnyvale" delta_msec="1487" seq="3"

2011-DEC-17 slotid="Location-Maps-US-MountainView" delta_msec="1445" seq="2"

2011-DEC-17 slotid="Location-Maps-US-SF" delta_msec="1465" seq="2"

2011-DEC-17 slotid="Location-Store-CA-MountainView" delta_msec="1445" seq="2"

2011-DEC-17 slotid="Location-Store-CA-SF" delta_msec="1245" seq="2"

2011-DEC-17 slotid="Location-Msg-CA-MountainView" delta_msec="1445" seq="2"

2011-DEC-17 slotid="Location-Msg-CA-SF" delta_msec="1245" seq="2"

i want to calculate a new field(avg_msec) based on the "slotid" we would like to calculate an average for all logs matching "Location-Maps" we would like to calculate an average for all logs matching "Location-Store" we would like to calculate an average for all logs matching "Location-Msg"

Can somebody help write the search string? Thanks!