Writing the Discussion Section
(printable version here)
The Discussion should be written after the Results section so that you have a good idea of what the experiment has demonstrated. The discussion section should definitely have a statement of your expected findings (Pechenik, 86). This should include your hypothesis and a brief statement about why these types of results are expected. There should also be a comparison of how your actual results related to your expected findings (Pechenik, 86). Here, you should state whether or not your results supported or didn't support your hypothesis. In addition, the degree to which the evidence supported your hypothesis should be stated. For example, were the results completely supportive, or were there variances?
There should be an explanation of unexpected results (Pechenik, 86). When looking for possible explanations, consider the following:
- Was the equipment used adequate for the task?
- Was the experimental design valid?
- Were the working assumptions made correct?
A common mistake that many writers make is to blame themselves for the unexpected results. Unless you actually made a mistake following the methods of the experiment, and could not go back and correct it, do not make up such errors to explain the variances you observe. Think about and analyze the methods and equipment you used. Could something different have been done to obtain better results? Another possibility to consider is if the experiment was conducted under factors that were considerably different from those described in the manual. Be sure to include ideas on how to test these explanations (Pechenik, 86). Briefly explain a way to test these possible reasons for unexpected results. For example, if there is a problem with the methods, maybe the experiment should be reproduced with an added step. Also, mention what kinds of experiments still need to be conducted in order to obtain more information.
The following text includes two samples of discussion sections of a lab report on enzymes. Italicized words are links to explanations of why that particular part of the introduction is important and what makes the sentences appropriate or in need of improvement.
Sample 1: The results of the first experiment supported the hypothesis that the rate of conversion of the substrate would increase with increased amounts of enzyme. We observed that Tube 2, which had the highest concentration of enzyme, catecholase, also had the highest absorbance level. Since absorbance is used as a measure of reaction, the greatest rate of conversion of catechol and oxygen to benzoquinone was seen in Tube 2. The high ratio of enzyme to substrate caused the absorbance to grow rapidly and then level off (see Figure 1). The tubes with lower concentrations of enzyme had lower rates of conversion, as expected. However, there were some unexpected results in Tube 2. Between the times of around 6 minutes to 8 minutes there was decrease in the absorbance. One explanation of this observation is that the settling of the substrate to the bottom of the test tube caused the enzyme to become less efficient since it could not attack the substrate as well. The settling reduced the surface area of the substrate that could be attacked by the enzyme. The tube was inverted and the substrate was stirred up, which caused a rise in the absorbance. Further experiments, involving the constant stirring of the solution, could be performed to test this possibility.
The folding and combination of polypeptide chains forms the specific, three dimensional shape of an enzyme. This shape is extremely important to the enzyme's catalyzing efficiency and many environmental conditions can affect the shape of enzymes and thus their efficiency. A range of pH values exists for all enzymes, between which they reach their maximum catalyzing action. This range is usually between a pH of 6-8. pH levels outside this range can denature the enzyme, thereby decreasing its catalyzing ability. The results we obtained supported this assumption for the catecholase enzyme. The catecholase samples in tubes 3 and 4 had similar absorbance rates and, therefore, similar enzyme activities. However, the pH of 4 in tube 2 corresponded to low absorbance and low activity of the enzyme in that tube. This is due to the fact that the acidic environment is harmful to the enzyme, and denatures it. Catecholase, an enzyme found in fruits in nature, is well adapted for efficiency in nature. Its range of optimal pH levels, 6-8, allows it to function in the varying pH levels of soil and those caused by acid rain.
Sample 2: Enzymes catalyze reactions by lowering the activation energy of the reaction. Catecholase, an enzyme found in potatoes, converts catechol to benzoquinone in the presence of oxygen. It would be expected that more benzoquinone would be formed in the presence of a greater amount of catecholase. This hypothesis was supported by the results obtained. The most enzyme was placed in tube 2. The absorbance was also highest for this tube. This means that the most product was formed in this test tube. In accordance with this, tube four, which had the least amount enzyme, also had the least amount of absorption. There were some unexpected results, but this is most probably due to human error; the absorbance levels were probably read wrong.
Enzymes are affected by the environment. The pH level of the environment is one factor that can alter enzymes. The rate at which the enzyme form product is slowed or sped up depends on how close to the norm the environment is. In the second experiment, the pH of the medium was different in each of the test tubes. The general trend seen in these reactions was that the more acid added to the test tubes, the less product formed. The more acidic solution caused the enzyme to work less efficiently.
Explanations of the Example Links
Results: This author does a good job of answering the questions that should be addressed in a discussion. For example, in the very first sentence he stated what he expected to find and also whether or not the results he obtained supported or failed to support his hypothesis. This is a good, strong way to start a discussion section. It starts off with the facts of the experiment and then later on, the author can move on to his opinions. (return to Sample 1)
Absorbance: A good discussion includes good ideas and also exact and detailed support of these ideas. In addition to starting off well, the author also goes on to explain the specific results of the experiment that support his hypothesis. This is what defines the strength of his discussion section. (return to Sample 1)
Explanation: After his explanation he presents the unexpected results and discusses possible reasons for this data. The author's explanation of possible reasons for unexpected results is good because it shows that he thought about the problems. He does not blame himself for the unexpected. Instead, he considers the methods used, presents a possible explanation, and then justifies his ideas. (return to Sample 1)
Catalyze: This author does a good job outlining his discussion; however, he is lacking the specifics to make a good discussion. The first two sentences are better placed in the introduction. However, he does state his expectations and whether or not his results supported these expectations. He could have made this part better by stating this more authoritatively, for example: "It was expected," and not, "It would be expected that." (return to Sample 2)
Unexpected results: The biggest problem this author had was explaining the unexpected results. He blamed himself, saying he read the equipment wrong and passed off the unexpected results as human error. (return to Sample 2)
Enzymes: This author does not develop his argument enough. One example of this is the affects harsh environmental factors have on enzymes. He could have stated how the acidity caused the enzymes to denature, thus creating less efficiency. (return to Sample 2)
All citations from Pechenik, Jan A. A short guide to writing about Biology. pp. 54-102, Tufts University: Harper Collins College Publishers. 1993.
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Below is a sample lab report assignment from a UW-Madison bacteriology course.
We will be using a format for the lab reports which is similar (but modified) to formats for scientific papers. That is, you must include an abstract, introduction, materials and methods section, results section, discussion, and literature citations. Your grade on the reports will depend on completeness, scientific accuracy and insight, organization, and writing skills. We will discuss this more in lab. We expect lab reports to be prepared using modern word-processing programs.
The format is as follows point totals for each section are for a 100 point report. For partial or 150 point reports they will be adjusted as needed.
1. Abstract of experiment. (10 points)
This is a summary of the basic content of the experiment. It should state the purpose of the experiment, mention the techniques used, report results obtained, and give conclusions. The point of the abstract is to give a concise summary of the whole report. The most common mistake that students make is not including summary data. Example:
Chromosomal DNA was successfully isolated from Bacillus subtilis strain 151 using a modification of the Marmur technique. Spectrophotometric analysis revealed some contamination with protein, but little RNA contamination. The pure DNA had a concentration of 1.05 mg/ml with a 10.3 mg total yield. The DNA was sterile, as judged by streaking onto penassay agar.
2. Introduction. (20 points)
An introduction gives focus to the report similar to the "Purpose" written in the lab notebook, but also should put the experiment into context and provide the reader with information necessary to understand the scientific basis of the experiment and the techniques used. In most cases, you should include background information on the organisms used and explain the theory behind the techniques. Much of the introductory material should be referenced and references have been put on reserve for you at Steenbock Library. You are encouraged to also search the library for other relevant references.
3. Materials and Methods. (30 points)
This is a section which will be a major deviation from scientific papers. Instead of asking you to tediously rewrite all your lab notes into a materials and methods format, we instead want you to include your lab notes in lieu of materials and methods. The lab notes should be complete, including all raw data, observations, calculations and appropriate graphs.
We do not expect (nor do we want) rewritten notes.
4. Results. (15 points)
Separate from the lab notes, include a section containing a summary of the final data, presented in a form that is most useful for interpreting the results. A short paragraph should be sufficient, along with any relevant charts and graphs labeled well. Remember to title and provide legends for all graphs and tables. The graphs and tables should be comprehenable independently of their association with the text.
5. Discussion. (25 points)
Discuss the experiment and the results obtained. This does not mean you simply report the results again, but rather interpret and discuss their significance. Results should also be compared with those in the literature, if possible. (Be sure to give proper citations). If problems were encountered during the course of the experiment, how might they be rectified in the future? Are there any other things we could do to make this a better experiment or to more specifically address the initial question posed? Are there any better techniques available that would allow one to more accurately generate data? Is there more than one way to explain the results? Your results may support your initial hypothesis, but there may be more than one conclusion that could be drawn from your results. Lastly, do not spend enormous amounts of time explaining data that cannot be explained!
6. Reference Citations
As required in all scientific literature, statements of fact, not considered "common" knowledge, must be properly referenced. Relevant articles for each of our experiments are on reserve in Steenbock Library.
Give complete citations of all literature cited in the report. What's complete? Here are some examples:
Articles in Journals:
Marmur, J. 1961. A procedure for the isolation of deoxyribonucleic acid from
microorganisms. J. Mol. Biol. 3:208-218.
Articles in Books:
Coakley, W.T., A.J. Bates and D. Lloyd. 1977. Disruption of bacterial cells. p279-341. In A.H.
Rose and D.W. Tempest (ed.), Advances in Microbial Physiology, Vol. 16. Academic Press, London and New York.
Department of Bacteriology
University of Wisconsin-Madison