David Auerbach has described an effect that he observed in samples in glass beakers placed into a liquid cooling bath. In all cases the water supercooled, reaching a temperature of typically  before spontaneously freezing. Considerable random variation was observed in the time required for spontaneous freezing to start and in some cases this resulted in the water which started off hotter (partially) freezing first.
In 2016, Burridge and Linden defined the criterion as the time to reach , carried out experiments, and reviewed published work to date. They noted that the large difference originally claimed had not been replicated, and that studies showing a small effect could be influenced by variations in the positioning of thermometers: "We conclude, somewhat sadly, that there is no evidence to support meaningful observations of the Mpemba effect."
In controlled experiments, the effect can entirely be explained by undercooling and the time of freezing was determined by what container was used. Experimental results confirming the Mpemba effect have been criticized for being flawed, not accounting for dissolved solids and gasses, and other confounding factors.
Philip Ball, a reviewer for Physics World wrote: "Even if the Mpemba effect is real — if hot water can sometimes freeze more quickly than cold — it is not clear whether the explanation would be trivial or illuminating." Ball wrote that investigations of the phenomenon need to control a large number of initial parameters (including type and initial temperature of the water, dissolved gas and other impurities, and size, shape and material of the container, and temperature of the refrigerator) and need to settle on a particular method of establishing the time of freezing, all of which might affect the presence or absence of the Mpemba effect. The required vast multidimensional array of experiments might explain why the effect is not yet understood.
New Scientist recommends starting the experiment with containers at , respectively, to maximize the effect. In a related study, it was found that freezer temperature also affects the probability of observing the Mpemba phenomenon as well as container temperature does.