Biases are elements of your research or experiment phase that can skew the results of your work toward a specific conclusion. This includes survey questions that present the survey taker with a clear (if false) choice, such as the survey question "Do you support green business practices or do you hate the world?" This student constructed the question on a logical fallacy, providing the survey taker with no real choice to make and invalidating the question. You can also cause research biases by artificially limited your testing process, such as selecting only men to take a survey, while stating in your research that your test subjects were chosen at random.
Limitations are the unavoidable shortcomings of your experimentation process, related to the methods you employ or the technology that you rely on. These include having only enough money to print out two dozen surveys or having to state that the microscope you used had a maximum magnification of 1,000x. While your 1,000x microscope is extremely high quality at the time you are working on your dissertation, if someone else reviews your dissertation later with a 5,000x microscope, he may learn something you could not have found.
You should avoid any biases in your research. Biases show a weakness in your research techniques and can invalidate the hard work you put into your dissertation. Review each question on your survey and make sure that there is no clear preference for one answer or another. Discuss your research methods with your dissertation chair if you have questions about any of your techniques, and request her opinion about any biases of which you may not be aware.
Reasonable limitations are perfectly acceptable in your dissertation, and within your dissertation you will include a special section to explain your limitations. Your limitations provide a foundation for future students to expand on your work by starting their research where your limitations halted yours. Every research project has reasonable limitations, but you can reduce your limitations by making use of the technology and facilities at your disposal. Use the best technology and methods at your disposal while you are researching.