date_trunc to aggregate values?

I am looking for suggestions on aggregation techniques using a timestamp =
column. In my case I have tried:

date_trunc('hour', derived_tsoil_fifteen_min_stacked.time2),

but date_truck only seems to aggregate the timestamp. I thought I could =
use=20

AVG(derived_tsoil_fifteen_min_stacked.value)

in combination with date_trunk, but I still get 15 minute values, not =
the hourly average from the four 15 minute records.

rowid                            date_truck        =
        time2                site    canopy    plot    =
variable    name            value    avg
2010-07-07_00:00:00_b4warm_a    2010-07-07 00:00:00    1    =
2010-07-07 00:00:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    21.06    21.0599994659424
2010-07-07_00:15:00_b4warm_a    2010-07-07 00:00:00    1    =
2010-07-07 00:15:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    20.96    20.9599990844727
2010-07-07_00:30:00_b4warm_a    2010-07-07 00:00:00    1    =
2010-07-07 00:30:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    20.88    20.8799991607666
2010-07-07_00:45:00_b4warm_a    2010-07-07 00:00:00    1    =
2010-07-07 00:45:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    20.8    20.7999992370605
2010-07-07_01:00:00_b4warm_a    2010-07-07 01:00:00    1    =
2010-07-07 01:00:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    20.72    20.7199993133545
2010-07-07_01:15:00_b4warm_a    2010-07-07 01:00:00    1    =
2010-07-07 01:15:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    20.64    20.6399993896484
2010-07-07_01:30:00_b4warm_a    2010-07-07 01:00:00    1    =
2010-07-07 01:30:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    20.55    20.5499992370605
2010-07-07_01:45:00_b4warm_a    2010-07-07 01:00:00    1    =
2010-07-07 01:45:00    cfc    closed    a2    tsoil_sc    =
tsoil_avg1_sc    20.47    20.4699993133545

I was tying to get two records out of this set, with the 'avg" column =
representing the mean of the first and last four of each 15 minute =
records.=20

Suggestions?