An additional factor is the type of interest in question. We can think of interests as scalar; crucial interests are weightier than important interests, important interests are weightier than replaceable interests, and all are weightier than trivial interests or mere whims. When there is a conflict of interests, crucial interests will always override important interests, important interests will always override replaceable interests, etc. So if an animal has an interest in not suffering, which is arguably a crucial interest, or at least an important one, and a person has an interest in eating that animal when there are other things to eat, meaning that interest is replaceable, then the animal has the stronger interest and it would be wrong to violate that interest by killing the animal for food if there is another source of food available.
Often, however, conflicts of interests are within the same category. The Inuit's interest in food is crucial and the explorer's interest in life is crucial. If we assume that the explorer cannot otherwise provide food for the hunter, then it looks as if there is a conflict within the same category. If you take the interests of an indigenous hunter's whole family into account, then their combined interest in their own survival appears to outweigh the hapless explorer's interest in continued existence. Indeed, if painlessly killing and eating the explorer were the only way for the family to survive, then perhaps this action would be morally condoned. But this is a rather extreme sort of example, one in which even our deepest held convictions are strained. So it is quite hard to know what to make of the clash between what a utilitarian would condone and what our intuitions tell us we should believe here. Our most basic prima facie principles arise and are accepted under ordinary circumstances. Extraordinary circumstances are precisely those in which such principles or precepts give way.
The multi-factor utilitarian perspective is particularly helpful when considering the use of animals in medical research. According to the animal rights position, the use of animals in experimental procedures is a clear violation of their rights—they are being used as a mere means to some possible end—and thus animal rights proponents are in favor of the abolition of all laboratory research. The utilitarian position, particularly one that incorporates some kind of multi-factor perspective, might allow some research on animals under very specific conditions. Before exploring what a utilitarian might condone in the way of animal experimentation, let us first quickly consider what would be morally prohibited. All research that involves invasive procedures, constant confinement, and ultimate death can be said to violate the animal's crucial interests. Thus any experiments that are designed to enhance the important, replaceable, or trivial interests of humans or other animals would be prohibited. That would mean that experiments for cosmetics or household products are prohibited, as there are non-animal tested alternatives and many options already available for consumers. Experiments to determine the effects of recreational drugs, cigarettes, and alcohol would also be prohibited. Certain psychological experiments, such as those in which infant primates are separated from their mothers and exposed to frightening stimuli in an effort to understand problems teenagers have when they enter high school, would also come into question. There are manyexamples of experiments that violate an animal's crucial interests in the hopes of satisfying the lesser interests of some other morally considerable being, all of which would be objectionable from this perspective.
There are some laboratory experiments, however, that from a multi-factor utilitarian perspective may be permitted. These are experiments in which the probability of satisfying crucial or important interests for many who suffer from some debilitating or fatal disease is high, and the numbers of non-human animals who's crucial interests are violated is low. The psychological complexity of the non-humans may also be significant in determining whether the experiment is morally justified. In the case of experimenting in these limited number of cases, presumably a parallel argument could be made about experimenting on humans. If the chances are very high that experimenting on one human, who is a far superior experimental animal when it comes to human disease, can prevent great suffering or death in many humans, then the utilitarian may, if side effects are minimal, condone such an experiment. Of course, it is easier to imagine this sort of extreme case in the abstract, what a utilitarian would think actually morally justified, again depends on the specific empirical data.