ere is the problem, as Dr. Linda Bartoshuk sees it: Say that two men, call them Richard and John, are both suffering from depression, and a researcher wants to find out if a particular medication will offer them relief.
Asked to rate the intensity of his depression on a scale of 1 to 10, Richard selects a 6. John, given the same rating scale, also picks a 6. But does he feel the same degree of depression as John?
Many researchers, said Dr. Bartoshuk, a psychologist at the Yale University School of Medicine and an expert on taste perception, assume the answer is yes, that, in effect, a 6 is a 6 is a 6.
But in fact, Dr. Bartoshuk said, nobody really knows, since depression, like many internal experiences, is subjective.
And therein lies an error that compromises many psychological studies, she believes, perhaps calling their findings into question.
Dr. Bartoshuk has described the error � which pops up in studies of everything from depression and taste perception to pain, prejudice, eating disorders and self-esteem � at several professional meetings. And she has written several papers detailing how the mistake is made.
But not all of her colleagues have reacted with gratitude.
"There seems to be a lot of venom," Dr. Bartoshuk said. "I've gotten nasty letters that basically said I was a moron."
As Dr. Bartoshuk describes it, the problem is simple but easily overlooked. It creeps in, she says, when researchers use a common measurement technique, the rating scale, to compare different people's subjective experiences. Undetected, it can produce distorted or even backward results.
The problem can be corrected if studies are properly designed to take account of the relative nature of the subject's responses, Dr. Bartoshuk said.
But in a review of eight volumes of the journal Physiology and Behavior, Dr. Bartoshuk said, she found the error in 17 of 68 human studies that used rating scales. In some cases, the mistake completely invalidated the studies' findings, she said.
Mapping the inner world is the goal of much psychological research. And for many researchers, rating scales offer a convenient method to explore what their research subjects are feeling and thinking.
In a typical study, subjects are asked to indicate, by locating themselves on a 9- or 10-point adjective scale, the intensity of a feeling or sensation.
A scale measuring hunger, for example, might ask the subjects to rate their appetite levels from 1 ("not at all hungry") to 10 ("very hungry"). An experiment on taste perception might invite ratings of the taste intensity of a particular substance from 1 ("very weak") to 9 ("very strong").
Rating scales are highly effective when researchers want to find out how the same subjects' feelings, attitudes or sensations change over time or in different situations. A psychologist, for example, might ask a group of subjects to rate how depressed they felt, give them six weeks of psychotherapy, and then have them repeat their ratings to see how helpful the therapy was.
The scales are also useful when researchers randomly assign subjects to different groups � giving each group a different drug, for example � and compare the results.
But many investigators also use rating scales for another purpose: they want to know how two groups of subjects, who differ in some way, experience things differently. A researcher might ask, for instance, whether men were more jealous than women, whether cocaine addicts craved cocaine more than smokers craved nicotine, or whether people with different psychiatric diagnoses suffered from similar or different levels of depression.
