When carrying out qualitative evaluation in chemistry, you are able to determine a reasonable amount of data in statistical form, in contrast to the explanatory form. The qualitative analysis provides you with a numerically based value of an experiment. But it cannot offer you explanations of why the chemicals being used in the experiment behave in a particular way. Chemists carrying out qualitative analysis therefore need a theoretical knowledge of the experiment they are performing.
Most chemists perform their qualitative evaluations in a lab. In most cases, the lab cannot offer an environment sufficiently resembling the environment in which natural reactions take place. This leads to creation of a control of laboratory conditions to resemble those of the real world's optimal conditions. As a consequence of trying to modify the conditions of the experiment, results can differ from those of experiments done in natural conditions.
Inaccuracies of data in most qualitative analysis experiments arise from the use of non-standard equipment. The end result of the experiment may greatly differ from a previous result if the experiment is performed using different equipment. Some equipment used in qualitative evaluation may also act as a limiting factor, such as when the material used to make lab equipment prevents some procedures from being carried out. For example, Pyrex glass might hinder a heat-related experiment because it takes a long time to heat.
For every chemical evaluation there is a stated hypothesis that awaits confirmation. A chemist performing a qualitative evaluation may well be influenced by his own view of the procedures, and may not sufficiently engage the participating subjects of the procedure. This can create a structural bias that makes some qualitative evaluation unreliable.