On the subject of gear retraction, the Boeing 737 FCTM states use the barometric altimeter only and not both the IVSI and baro altimeter indicating, as proof of positive rate of climb before gear retraction is initiated. Why is this so?
Commonly observed in simulator training is the PM calling "positive rate" when in fact the aircraft is barely airborne, and the PF calling "Gear Up" based on the PM's call rather than double checking his (the PF's) own altimeter is rising. This quirk is most often seen when the Non-Normal exercise is captain's static vent blocked before take off. In his case the F/O calls "Positive rate" on his own altimeter while the captain's altimeter and IVSI don't move due to the blocked static vent.
But the very action of calling "Positive Rate" by the F/O triggers the Pavlov's Dog reaction by the captain of answering "Gear Up" to the call - not to his instrument readings.