Knowledge based on individual stories rather than hard data is called anecdotal information. This information can be collected and shared by practitioners or (as often happens with PHAs) the people using the treatments. Anecdotal information is an important component of both complementary and conventional medicine. In conventional medicine, such observation may reveal new uses for existing treatments or identify unforeseen side effects.

In complementary medicine, anecdotal information is often recorded and compiled to form a base of information about the likely outcome of a treatment. Anecdotal information has limitations. It is based on the experience of individuals; how these experiences apply to others is often difficult to judge.

Although much of Western medical practice was developed from anecdotal information, the current standard for a Western medical treatment is a double-blind placebo-controlled trial. In such a study, a group of people with the same medical condition believe they are being given the same treatment. Placebos (fake treatments) are used by some trial participants, but no one knows who is getting the real treatment. The study is called double-blind because even the physicians and researchers who collect the results are not told which participants received placebos. This method is intended to eliminate biases based on the expectations of researchers and participants and to gather statistical evidence about how often we can expect the treatment to work. Some complementary therapies can and are being tested in double-blind placebo-controlled trials. This is particularly true outside of Canada. Unfortunately, various factors often hamper complementary therapies trials:

Money Practitioners and producers of complementary therapies rarely have the financial resources of a drug company. Even when they do, most complementary medicines can't be patented, so there is less financial incentive to pay for trials.

Skepticism Western scientists skilled in performing controlled clinical trials are often skeptical about complementary medicines. Due to this skepticism, trials of complementary therapies do not build a researcher's prestige in the same way that a typical drug trial might.

History Complementary therapy practitioners and users have not participated in many controlled clinical trials. As well, practitioners schooled in medical systems with established bodies of knowledge (such as traditional Chinese medicine) may see little need to re-examine these therapies to comply with Western medical standards. Western medical researchers, on the other hand, may see little need to study complementary therapies when a Western medical treatment exists.

Pure substance A controlled trial requires a purified, consistent dose of the treatment. In the case of some complementary therapies, this purified form is not available. In others, practitioners and users believe the therapy is most effective in its natural "unpure" state.

Some solutions to these problems are emerging. Governments are now more willing to dedicate resources to the study of complementary therapies. For example, the federal government of Canada has targeted funding for research on natural health products through the Canadian Institutes of Health Research and through the Natural Health Products Directorate. Other research funds are also targeting complementary therapy research. In some cases, trial methods may need to be adapted to study complementary therapies effectively. All of these efforts will require collaboration between Western scientists and complementary therapy practitioners to produce reputable results.