It dawned on me that both bottles and diapers are both indirect ways of measuring an 
infant's intake and subject to similar biases.

So, for instance:

If you pump milk and offer it to a baby in a bottle you might have the following potential 
MEASUREMENT errors:

1) Loss of milk via dribble (which would OVERestimate intake compared to direct test 
weighing)
2) Differences in ability to feed from breast and bottle (which would most often 
OVERestimate intake from the breast because you can force feed with a bottle, but 
sometimes might UNDERestimate intake from the breast with babies that don't feed well 
from a bottle)  --- this is the biggest flaw I think in the pump and put it in a bottle 
measurement approach
3) If you are not weighing the bottle, you will get a SLIGHTLY higher measurement error 
by observing the volume than weighing depending on the volume increments on the 
bottle.
 
If you weigh a diaper as an indicator of intake you might have the following potential 
MEASUREMENT errors:

1) Loss of urine and/or stool via leakage from the diaper.
2) Potential evaporation of water from the diaper (depending on length of time between 
when the output occurred and the diaper was weighed).
3) Lag time between when the milk went it and the product of that milk went back out.
4) Evaporative water loss which is compounded by 3).  If the measurement of intake is 
direct and immediate there is less time for evaporative water loss to occur than if the 
indirect measurement of intake occurs many hours or even sometimes a day later.
5) Activity level of the infant --- which might also impact evaporative water loss.

It also dawned on me where weighing a diaper might be pragmatic and helpful which 
would be when the baby is tube fed in an incubator and is not taken out for feeds.  That 
might be less disruptive in a NICU situation.

Best, Susan

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