Recording what you found out

Documenting what you measured during an experiment is, arguably, the most important part of an experimental record. Ensuring the details of when, how and why the experiment was performed are important for ensuring rigour of information, but without the measured or observed data being recorded in a way it can later be interpreted, the whole process of performing the experiment would be pointless.

It is worth pointing out at this stage the difference in using the terminology “results” and “data”. Often incorrectly used interchangeably, these terms do, in fact have subtle but definite differences and you should make efforts to use language precisely [1] to avoid ambiguity. Experimentalists like things to be clear and don’t like things that cause confusion. Data is the raw measurements or observations taken from an experiment, where as the result is the outcome from the experiment that can be interpreted from the raw data.

For example, consider an experiment to determine the specific heat capacity of water by recording the amount of energy it takes to change the temperature of 1 kg. You note down the following in your experimental record:

The table above, containing the numerical measurement of temperatures and energy, which you read directly from the instrumentation, is the data. The results are the processing of that data to determine the specific heat capacity.

Raw data

It is important to note that you should always record data in its raw form. This is to say it should be written down in exactly the same format as it is displayed on instrumentation before it is manipulated by any kind of unit conversion or other calculation, and do not round up or down in the first instance. If you are recording a length with a rule that uses centimetres, write down the data in centimetres. If you are using a depth gauge the displays in millimetres, write down the data in millimetres.

There are two reasons for recording raw data. Firstly, if you are processing the data in some way and you get it wrong, there is no way to track back to the original value and identify the mistake. For example, if you measure a volume in millilitres, but know to process the data you want it in meters cubed, you might be tempted to write down only the converted figure in your experimental record. If you did this, and made an error, the original reading is lost forever. Secondly, even if you perform a calculation correctly, it can remove the ability to quantify the uncertainty during error analysis. For example, if you were recording the difference in volume between a start and end point, you should record both the start and end value, rather than performing the subtraction and writing down only the final result.

Recording uncertainty

No measurement is ever completely precise, as is discussed in greater detail in the section Handling error and uncertainty. Uncertainty could occur because of limitations with the precision of the equipment, variation of measurement with time or through assumptions that are being made. Regardless of the reason, it is critical to capture this uncertainty in your experimental record, as this will be needed to interpret the data to find the significance of your result.

The easiest way to remind yourself to do this is to get into a habit that, for every number you write down in your lab book from a measured you have made during your experiment, you also write down the minimum and maximum value that the value could be based on the uncertainty. For example, if the measurements vary with time, then include the variation, e.g. if a thermocouple reports that a temperature varies between 38.64 °C and 39.17 °C then record this range, exactly as you observe it. So the record from the specific heat capacity experiment should have actually been recorded like this:

It can be tempting to predict the uncertainty that for all your readings and note this once in your record. This could be based on the equipment precision, for example, if you are measuring an electrical current, and the ammeter is precise to 0.01 A, you simply note this down in the experimental record and say it applies to all readings. However, it is more than likely that the uncertainty will not be the same for all readings during the experiment, so it is unwise to adopt this method. Besides which, you should have recorded the accuracy of the equipment’s scale in the procedure section.

If you are reading a laboratory notebook where the uncertainty hasn’t been specified explicitly, the precision of the measurement can be inferred from the precision of the written number. For example, if a temperature appears as 39°, it implies the measurement is 39 °C ± 0.5 °C but if it was appears as 39.0 °C, it implies the measurement is 39 °C ± 0.05 °C.

Use a table

There isn’t a requirement to record numerical data in a table. However, in the overwhelming majority of cases the most appropriate method to record numerical data is in a table. If possible, you should construct your table in advance of executing the experiment, so while involved in performing the procedure you can simply fill in the correct boxes. It is also acts as a handy reminder of what you are supposed to be measuring, as anything you miss will appear as an empty box. Because you plan your experiment in advance and know what you plan to measure, at least for initial findings, this is normally possible.

As with all tables, all columns should have headings and all numbers should have unit associated, which could be in the column heading but needs to be clear that it isn’t part of the title, by, for example, putting the units in brackets. The meaning of rows and columns should be clear. If you have set up a notation scheme in your procedure or equipment section, such as referring to a particular thermocouple as T1, you can use this as your column header.

But wait! What if I’m not writing down numbers?

When it comes to recording what you found out there are two instances where you will not be writing down numbers. The first is if you are recording qualitative data and the second is if your data is stored elsewhere, such as digitally.

Qualitative data from an experiment you want to record could be an observation, something interesting or pertinent to the experiment or some visual measurements. Noting the time (and date, if not elsewhere on the record) of any observation is useful, so that you can accurately reconstruct a series of events. You could write something down as a simple description, such as “at 12:05 pm a person entered the room creating a gust of air that passed over the sample” or “This result may have been influenced by a small bump to the desk, at 09:20 am, with the measurement was being taken”.

The data you are capturing might not be numerical. If this is the case, do not be afraid to sketch what you see as this is a useful and efficient way to record information. For example, the data in following petri dish is visual data that needs to be captured:

To make things clear, sketches should be surrounded by information, such as the setting used or labels of various parts of the diagram. Sketches don’t need to perfectly neat and you don’t need to be a great artist. They should simply be clear and legible to convey only what is important to the reader. Because of the ability to capture the essence of a visual results and the ability to quickly annotate, sketches can be of more use than photographs.

For more sophisticated experiments you may collect data that can’t be recorded the lab book, such as the picture from digital thermal imaging camera or sensors that are data logged on a computer. The key to keeping a rigorous record which include digital information is to ensure there is a way to connect the digitally stored information to the written information in the lab book. There are no specific conventions on how to do this, but noting down the date and time the file was created as well as the filename and device name where the digital files are stored, is a convenient method as most digital data is time stamped when it is created. There may be other metadata that is useful for locating your particularly data acquisition. Just make sure that it is clear which information from that lab book applies to each file that is digitally stored by noting the existence of the file in a suitable place and notating it correctly. For example, here is a table of results including how some pictures from a camera can be linked to the information written in the experimental record.