In an effort to convince people to join the study, the researcher himself may pose a threat to the validity of the study if he has affected the emotional state of the patient. If the researcher had to push a patient hard to join the study, that patient may have adverse physical or emotional effects when around the researcher or performing the study.
In the cases of qualitative studies (rather than quantitative studies), no numerical value can be placed on the patient's results. Instead, the patients' subjective well-being is recorded with adjectives that can mean different things to different doctors. Those reviewing the study without the input of the data gatherer may interpret the outcomes of the study differently.
In some instances, such as a diethylstilbestrol study done in the 1950s, doctors administer drugs to patients without their knowledge or consent to study the effects. While highly unethical, doctors have chosen to do this in the past for fear of not being able to secure enough test subjects to validate their study. According to the Journal of Nursing Scholarship, the patients in this particular case suffered high rates of cancer nearly 20 years later.
Doctors or nurses seeking acclaim may wish to publish falsified results if the study does not necessarily turn out the way they had anticipated. Publishing a scientific article about their work in a nursing or medical journal will draw attention and potentially more monetary funds to their work, especially if the results are uplifting in nature.