Guide to Writing a Formal Physics Lab Report

 
submitted by Chad Orzel, Physics Department, Union College

A formal lab report is essentially a scaled-down version of a scientific paper, reporting on the results of an experiment that you and your lab partner(s) have carried out.  As such, the key sections of the report are directly analogous to the sections of a formal scientific paper.  In the order in which they appear, these are the following: 

Abstract       The abstract is a single short paragraph stating the important results of your experiment, including
                      the numerical values, with appropriate units and uncertainties, and the most important conclusions
                      drawn from the experiment.

Introduction  The Introduction gives the important background for understanding the experiment, including both the
                       motivation for making the measurement and a complete description of the theory underlying the
                       measurement, with all the relevant equations.   
Procedure      The Procedure section gives a complete description of the important measurements you made, and
                       how you made them.  It is a description of what you did, and is not to be written in the style of
                       instructions to someone else. 

Results          The Results section presents the important experimental findings, including figures and tables
                      containing the date you collected, and text explaining the significance of the results.  The Results
                      section is not merely a collection of data tables and figures, but must include prose paragraphs as
                      well.   

Discussion and Conclusions   The Conclusion of the report explains the conclusions that you can draw from your
                      measurements--whether they agree with theoretical predictions, what they mean for applications of
                      the central physics principles, and what further experiments are suggested by your findings. 

In the following pages we will explore these sections in more detail, explaining the key elements of each section, and how they should be presented.

A Note on Writing:
        One of the most common complaints about the writing of lab reports, and especially the grading of lab reports, concerns the importance of writing.  A lab report, like a scientific paper, is first and foremost about communication, conveying your results to the reader, and as such proper writing is essential.  You can be a brilliant scientist, able to produce great results in the lab, but if you are unable to communicate those results effectively to another person, all your lab skills are worthless. 

The Lab Report Outline is intended as a guide to writing your lab report.  lists the sections of a formal lab report and shows various elements which need to appear in each section.

 

General Writing

Taking the most controversial element first, at least one-third of the grade for each lab report will be based on the general quality of the writing. This includes elements like grammar, spelling, and proofreading.

Calling this “wildly unpopular” would be an understatement, and students almost always react to this section by saying “Why do you grade on writing? We’re scientists and engineers, not English majors.” This attitude is dead wrong: clear communication is at least as highly valued in the sciences as in the humanities.

As a scientist, you can go into the lab and take data worthy of a Nobel Prize, but if you can’t explain the results of your experiments clearly and concisely in written form, you may as well not have done them. The key to all of modern science is reproducibility— for a result to be accepted as the correct result, other experimenters need to be able to reproduce the result. For that to be possible, you need to be able to explain to other researchers all around the world what your results were, how you got those results, and why those results are important. If you can’t write clearly, you’ll never succeed in communicating your results well enough to get the credit you deserve.

In a similar vein, an engineer is expected not only to design and develop useful technology, but also to convince people that his or her designs are the best. You can have a wonderful design for a cell phone that turns into a submarine, but if you can’t explain in writing how it works, what it’s good for, and why your cellular submarine design is superior to all other cellular submarine designs, you’ll never get anyone to buy it. All the technical skill in the world is useless without the ability to communicate your results to others.

The key to good writing is organization. A lab report, like a scientific paper or an engineering proposal, should have a clear and logical flow of ideas: first explaining the motivation of the experiment, then the procedure, then the results, then the conclusions drawn from those results. The reader should be led smoothly from one idea to the next, not tugged erratically back and forth between procedure, results, motivation and conclusions.

On a finer level, each sentence and each paragraph should have a clear point, and serve to advance the argument being presented. Writing is more than simply stringing together a disjointed collection of unrelated thoughts. “Stream of consciousness” lab reports are confusing and difficult to read, and create the impression that you don’t actually know what you’re talking about. Such an impression would be disastrous for a scientific paper or an engineering proposal, and will accordingly be marked down in a lab report.

An even more basic element of good writing is proofreading. There is no better way to make yourself look foolish than to turn in a written document with a huge, glaring spelling error in the first paragraph (especially in the current age of automatic spelling checkers in word-processing programs). Before you hand a lab report in, read it over, or have your lab partner read it over. Make sure the report makes sense, and that all the words are spelled correctly and used correctly. The spelling check on Microsoft Word (or any other word processor) won’t catch typos which end up matching real words (“precious” for “previous” and “preformed” for “performed” are common errors of this type).

 

Abstract:  Summarize your experiment in one short paragraph.  State the purpose, the experimental method, and the result.  Be concise--for example, often just giving the name of the method is sufficient.  If you have made a quantitative measurement of some quantity, state the result of the measurement, with uncertainty. Remember to  include the appropriate units on any results that you present.

 

Introduction:  Discuss any relevant theory and/or motivation for the experiment.  This section serves primarily to help the reader understand the significance of the experiment and all the issues that are later addressed. The main questions to be addressed in this section of the report are “Why are you doing this experiment?” and “What do you hope to find?”

            (Please note that “We’re doing this experiment to illustrate the concepts discussed in class,” while true, is not an adequate answer to the first question. When writing the report, pretend that you have chosen to do the experiment of your own free will, motivated by scientific curiosity, rather than having the experiment forced upon you by the inhuman taskmasters of the Physics Department.)

If the experiment is designed to test a particular physical theory discussed in class, you should describe both the general theory and the particular prediction you’re attempting to check in the Introduction.  This section is often the most difficult to write, and you may want to try writing this section last, since you want to be sure to introduce any important concepts that are needed for your discussion in later sections. 

 

Experimental Procedure:   Describe your set-up (drawings are usually needed) and the method used.  Do not just restructure the instructions in the lab hand-out, and do not assume that your reader has read the lab hand-out.

Make sure you describe the apparatus before  referring to parts of it. A Procedure section which starts out “We moved the cart back and forth on the track and recorded the position with the sensor” will be incomprehensible to a reader who was not in your class. You need to tell the reader that the apparatus consisted of a cart, a track, and a sensor, and also what kind of cart, track, and sensor you used. Including a sketch of the apparatus is not sufficient description; you must also describe the apparatus briefly in words.

Do not include unimportant details, such as where a particular switch is, what combination of keystrokes and mouse-clicks you use to do something in Science Workshop,  or how you line up your head to see that something is aligned.  In particular, you do not need to include steps like “Then we made a graph of position vs. time and printed the graph out”--  attaching the printed graph of position vs. time to the lab report is sufficient. It’s also not necessary to identify the software packages used to generate graphs and tables, though it is important to identify the software used to acquire data. For example, “We recorded the position as a function of time using the Science Workshop package” is important, while “We entered numbers into a data table in Microsoft Excel” is not.

Explain what you did in the course of the experiment, but don't write the procedure section like an instruction manual. Use the most direct descriptions possible, writing in past tense and active voice. For example, it’s better to write “We measured the length of the track using a meter stick” than “The length of the track was measured with a meter stick” or “Use a meter stick to measure the length of the track.”

Include measurements that are related to the limitations of the experiment, such as a measure of the amount of error in a particular measurement.  You will discuss these errors in more detail in the Results section, but how you measure them and what you measure them to be is relevant to the procedure and so needs to be discussed in this section.  They belong in this section because they give a quantitative measure of the accuracy and reliability of the results.

 

Results:  Present your data and calculations.  This is the meat of your report. 

First present the raw data.  Numerical data should be listed in a table and the table referred to in the text; graphical data (for example, position vs. time plots from Science Workshop) should be presented as figures and referred to in the text.  Be sure to include uncertainties in any measured quantities. 

After presenting the raw data, discuss any calculations that you made from that data.  If there are results of calculations that would be best presented in a table, make sure they are clearly distinguishable from the raw data, either by putting the processed data in a separate table, or by clearly labeling the columns. If there are results that would be best presented in a figure, label the figure clearly, and be sure to refer to it in the text.  Be sure to label the tables and figures and to refer to them in the text by name (e.g. “Figure 1”, “Table 2”).  Don't include a figure without discussing it in the text.  Explain the relevance of the figure, and what it tells you about the experiment. 

Be sure to include an uncertainty with every measurement.  In general the discussion of the measurement of the uncertainty (that is, how you determine the value of the uncertainty) is given in the experimental procedure section, but you still need to give the uncertainty (after a plus-minus sign) when presenting the results. Say “We measured the mass of the cart to be 1.02 ± 0.3 kg,” not “The mass of the cart was 1.02.”

Discuss your errors in this section.  Discuss the sources of error, both random and systematic, and how the errors affect your results.  Do not put off the discussion of the error until the Conclusion section. 

 

Discussion/Conclusions:  If there are further interpretations of the results or significant implications to be discussed, such a discussion should occur in this section.  You must also summarize the main results of the experiment.

This is where you should discuss the implications of the comparison between experiment and theory (Does your measurement agree with the theoretical prediction? If so, what does that tell you? If not, why not?) , or between two different methods of measurement (If you measured the same quantity in two different ways, which measurement was more accurate?).  Address any additional ideas you have about the experiment, such as improvements that could be made, or how the experiment relates to the material discussed in class. Use this section to be creative, wax philosophic, scale lofty heights of rhetoric.

 

Additional guidelines:  There are some more general mistakes in writing reports that appear to be fairly common:

Error in Uncertainty
The most significant common mistake is stating "the percent error" as the "uncertainty" in your result.  Many students calculate the percentage difference between their result with the "correct" result.  This is NOT the way to calculate uncertainty. 

You should imagine that the lab experiment is an actual scientific problem to be investigated. The whole reason for doing the experiment is that you don't know the correct answer and want to determine it. The uncertainty is a measurement of how well you’ve done your measurement, and how much confidence another researcher should have in the result, not a measure of how close you’ve come to a hypothetical “correct” answer. By quoting a “percent error” from some “correct” value, you’re saying both that your measurement is wrong, and that the experiment was pointless in the first place: if you knew the correct answer, you wouldn’t need to do the experiment at all.

You can (and often should), however, discuss "the percent disagreement" or “percent difference” between your result and the theoretical prediction.  Such measurements are an important part of determining the usefulness of a particular theory, but are not related to the uncertainty associated with a given measurement. This may seem like a trivial semantic distinction, but it’s not.

Figures and Headings
Another very common (and annoying) mistake made by students is neglecting to label the figures and tables and/or not referring to them in the text.  Figures and tables are crucial components of a science paper and should be able to stand by themselves.  They should include a short caption or descriptive label, telling the reader at a glance what is being presented. If multiple quantities are plotted in a single figure, the different data sets should be clearly distinguished (using different symbols or line styles, with a key to the symbols or styles included in the caption or the figure itself).  Figures and tables should also be referred to at the appropriate points in the text, to be sure to draw the reader’s attention to the data.

Figures and tables should be assigned numbers according to the order in which they appear in the text, and should be referred to by number (e.g., “A graph of velocity vs. time is shown in Figure 1” or “The data for the second trial are shown in Table 2.”). Tables and figures are numbered separately—the first figure is “Figure 1,” and the first table is “Table 1” regardless of where they appear in the text.

The figure caption should be a short (one or two sentence) description of what is in the figure. If multiple quantities are plotted, the caption should identify the symbols used for each. Figures which show the arrangement of experimental apparatus should mention the most important components in the caption, and if curve fits are used, the relevant fit parameters should be mentioned in the caption (e.g.: “The solid line is a linear fit to the data. The slope of the line gives the mass of the cart, m=1.007 ± 0.003 kg.”)
 

Equations: Equations that appear in the text should be put on their own line, and centered in the text. When multiple equations are used in a report, you should number them according to the order in which they appear, and refer to them by number (e.g.,“As we see from Equation 1, the force is proportional to the acceleration”).

When you introduce a new equation, you should be sure to define all the symbols in it.  For example, if I want to refer to Newton’s Second Law, I would write: “The motion of an object is determined from Newton’s Second Law,

                                                Fnet = m a                                             (1)

where Fnet is the net force acting on an object, m is the mass of the object, and a is the acceleration of the object.”

 
Marketing:   One last piece of advice:  write the report honestly--don't try to over-sell your results.  If the results are only marginally significant, you are better off stating clearly in the report that you believe this to be the case.  Others may still find it interesting enough to follow up on your experiment.  Over-selling of marginally significant experiments suggests that the author of the lab report doesn’t actually understand the material that he or she is talking about.

Academic Honesty: You will work on the lab experiments in groups, and you are always free to discuss your results and the interpretations of those results with your lab partners.  Indeed, you are encouraged to discuss your results, and even discuss how best to present the results.

What you hand in your lab report, however, must be your own work, and only your own work. You are not allowed to copy sections of a report from another person, or to write sections of a report with or for another person. You can proofread your partners’ reports, and ask your partners to proofread your report, but any changes made to the text must be made by the person who will hand the report in for grading (i.e., you can neither offer nor accept verbatim re-writes of paragraphs in your report).

Lab reports that are wholly or partly identical will receive a grade of zero, and be referred to the Deans as plagiarized.  If you have any questions regarding the limits of acceptable collaboration on lab reports, ask your lab instructor.  Do not assume that the standards used in another class will apply here, and do not attempt to guess what is acceptable and what is not.



Submitted by Chad Orzel, Physics Department, Union College.