Humans will try to keep their personal worldview and prejudices out of their work, but this sometimes fails. When a researcher or scientist influences results in order to portray a particular outcome, it is called research or experimenter bias.
Sometimes bias shows up in the design of the experiment. This happens when a scientist knows that the experiment was conducted in such a way that it eliminated data that might contradict the conclusion he or she was seeking.
Other times bias can show up when analyzing the data - if some of the data are removed from the dataset before conclusions are made. It is always important to know who conducted the research you are studying to see if there may be any potential bias found in the report.