In a Latin-square design, 16 chronically agoraphobic outpatients had four sessions of long or short flooding in fantasy and in practice. Fantasy sessions were given by tape recorder and produced minimal arousal with little improvement. Two hours of flooding in practice reduced phobias significantly more than did four half-hour periods of the same procedure in one afternoon. During long flooding in practice, heart rate and subjective anxiety decreased more over the second than the first hour of exposure to the real phobic situation; though treatment was directed towards top hierarchy items, bottom items improved more. Patients' expectation of improvement at the start of treatment did not correlate with outcome.
Stern R, Marks I. Brief and Prolonged FloodingA Comparison in Agoraphobic Patients. Arch Gen Psychiatry. 1973;28(2):270-276. doi:10.1001/archpsyc.1973.01750320098015