Science Buddies: "Ask an Expert"

How should you write "assumptions and limitations"?

Hello mAlbright34.

First of all, be sure to read any instructions provided by your Science Fair. Details vary form one Fair to another.

The idea behind an "assumptions and limitations" section is for the author to point out issues that might affect the experiment's result. For example, if an instrument being used to detect arsenic was able to detect only a certain lower limit, then that would be a limitation, and the author might say,

• "The arsenic detector used in this experiment was unable to detect arsenic concentrations below two parts per million. While we did not detect arsenic in several samples, the sample might have contained arsenic at levels below two parts per million."

In the same way, the author should discuss assumptions. For example, if perishable samples had been stored for a while before measurements were made, the author might write,

• "We stored the water samples on ice and transported them to our lab as quickly as possible. However, bacteria may have grown in the samples after we collected them, and we are unable to separate that bacterial growth from the level of bacteria present when the sample was taken."

At the end of the section, you can discuss the issues mentioned. For example,

• "While it is possible that some bacteria grew after sampling, the type of bacteria we found do not grow rapidly at 0 deg C, so we believe any error due to this factor is small."

I hope this helps, WW

The explanation above is very clear, but I would like to add some examples that came to mind when I first read your question. Often, in school, students make assumptions when doing labs. Physics in particular requires using blocks of different masses: 1 g, 20 g, 250 g, etc. Usually, it is just assumed that these masses are correct, and are thus recorded, although they might be slightly lower or slightly higher. Assumptions like these can be sources of error, which is certainly something to discuss in a paper describing an experiment, whatever it may happen to be about. A limitation could be that the equipment, e.g. a spring scale, is defective or too crude to produce accurate results.

No matter what assumptions or limitations that you come up with, you should remember to elucidate exactly how they affected the results and what could be done to improve the experiment if it were to be repeated in the future!