InLab: the lab procedure
1. Setting up the lab:
Before you start the lab, review the objectives and the procedures you
will follow. Take detailed notes as you gather your materials, set up
your lab, and calibrate instruments. These notes will help you document
your experimental protocol, which you can use later when writing the Methods
section of your lab report.
- List the materials you will be using. If using a specific instrument,
you may want to make a sketch with appropriate labels for your lab notebook
and for your lab report. If you have any questions about how to use
any of the materials or equipment you will need for your lab, make sure
to write them down. As you proceed with the lab, you will most likely
find the answers. These notes will help you later when you write your
lab report.
- When using laboratory equipment, there are many sources
of error or uncertainty (see definition below under step #3) that
may arise. Make sure to note these in your lab notebook. You will need
to refer to them again when you write your lab report.
- In your lab notebook or manual, identify the types of data, you will
be collecting during the lab, such as drawings, lists of physical properties,
or descriptions of chemical reactions. Identifying types of data will
make you ready to record your data properly in your lab notebook or
manual.
Go to the on-line version of this document to see example lab
notebook pages.
2. Preparing to collect data:
If you are collecting quantitative
data, identify the variables
and units
of measurement and create a table or set up a spreadsheet
(for help with creating tables or setting up spreadsheets, go to the on-line
version of this document or go to the LabWrite Resources homepage). If
you are collecting qualitative
data, determine the kinds of data you will be collecting and then
prepare appropriate materials for recording observations (drawings, tables
for observations, photographs, etc.). Read the lab manual to see what
kinds of data you are being asked to record and be sure that you are ready
to record the data in the appropriate form when you begin the lab procedure.
Whatever the data may be, it is very important that you decide whether
a table
or spreadsheet is best for organizing your data so that you can refer
to them later on when writing your lab report. (See below for definitions
of underlined terms.)
Quantitative Data:
Quantitative data are data for which the scale of measurement has magnitude
and is either interval or ordinal. Quantitative values are continuous,
so each possible value may be greater or less than any other value.
Ordinal data has an order but the distance between values does not have
precise numerical meaning. For example, rank in a graduating class or
ranking of runners in a race. Interval data uses a scale that has a
specific numeric distance between values: it has a unit of measurement.
For example, the time which the runners ran the race is interval data.
Qualitative Data:
Qualitative data are data for which the scale of measurement is a set
of unordered categories called a nominal scale. For example, types of
trees, types of compounds, etc. Qualitative variables are considered
discrete variables, because they vary in some quality but not magnitude--one
category is not greater than the other.
Qualitative data are based on observations that do not lend themselves
to numerical measures. They often call on scientists to make judgments
about data. Qualitative data can be a record of differences in qualities,
such as color change of a solution or the vitality of a plant over time.
These kinds of data may be recorded in a table. Or qualitative data
can be pictorial, such as a cell seen through a microscope or the petal
structure of a flower. These kinds of data may be recorded in drawings
or photographs.
Variables:
A variable is what is measured or manipulated in an experiment. Variables
provide the means by which scientists structure their observations.
Identifying the variables in an experiment provides a solid understanding
of the experiment and what the key findings in the experiment are going
to be.
To identify the variables, read the lab procedure described in the
lab manual. Determine what you will be measuring and what you will be
manipulating for each measurement. The first of these are the dependent
variables and the other is the independent variable (see definitions
and examples below). Write down the dependent and independent variables.
A dependent variable is what you measure in the experiment and what
is affected during the experiment. The dependent variable responds to
the independent variable. It is called dependent because it "depends"
on the independent variable. In a scientific experiment, you cannot
have a dependent variable without an independent variable.
An independent variable is the variable you have control over, what
you can choose and manipulate. It is usually what you think will affect
the dependent variable. In some cases, you may not be able to manipulate
the independent variable. It may be something that is already there
and is fixed, something you would like to evaluate with respect to how
it affects something else, the dependent variable.
It is possible to have experiments in which you have multiple variables.
There may be more than one dependent variable and/or independent variable.
Usually, you choose one independent variable at a time and observe its
effect on one or more dependent variables.
Unit of Measurement:
A standard of basic quantity or increment by which something is divided,
counted, or described, such as ml, kg, mm, m/s, °F, etc.
Creating a Table or a Spreadsheet:
A table provides a very convenient tool for organizing the data you
collect in your lab. You can quickly draw a table on a sheet of paper,
you can make one with a word processing program, or you can generate
one with spreadsheet software. Using a hand-drawn table in the lab also
allows you the flexibility of entering the data into a spreadsheet at
a later time. The chief advantage to entering data in a spreadsheet
is that you can easily convert it not only into a table but also into
all sorts of graphs.
Use this guide to figure out whether or not you should use a table or
a spreadsheet for recording your data in the lab:
If you do not have access to a computer with spreadsheet software in
your lab, then you should create a table. You can use the data in the
table to generate a spreadsheet later, if necessary.
If you know you will need to create graphs for your data and have access
to spreadsheet software in the lab, then use the spreadsheet.
If you are not sure what form, table or graph, you will be using to
report your findings and it is convenient to use a spreadsheet, then
use a spreadsheet.
If creating a spreadsheet in the lab will take too much lab time, then
use a table and create the spreadsheet later.
3. Collecting and recording lab data :
Carefully follow the experimental protocol. As you conduct your experiment
and record your data, take notes on what you are doing and on any changes
in the procedure. Taking good notes will help you recall the lab later
on when you are writing your lab report. It's also important to note any
problems with the procedure or deviations from the established protocol.
Even if you are following the protocol in a lab manual, sometimes you
will set up and run things differently. It could be that the materials
specified in the lab manual were not available precisely as indicated,
or perhaps your lab instructor decided to change the protocol somewhat.
As you record your data, you should be asking yourself various questions:
What are the relationships
among the variables? Do the data behave in the way that you had anticipated?
If not, why not? If the data make no sense, you may need to consider sources
of uncertainty once again. Sources of uncertainty may affect the accuracy
and precision of your experimental data (See below for definitions
of underlined terms.)
Relationships Among the Variables:
Since dependent variables "depend" on independent variables,
there has to be a relationship between the two. The relationships between
the dependent and independent variables are what is described in the
hypothesis. So it's important to determine what those relationships
are in order to see whether or not the hypothesis has been supported.
Sources of Uncertainty:
In science, a source of uncertainty is anything that occurs in the
laboratory that could lead to uncertainty in your results. Sources of
uncertainty can occur at any point in the lab, from setting up the lab
to analyzing data, and they can vary from lab to lab. This is why it
is so important to keep detailed notes of everything you do in the lab
procedure and any problems you encounter. Try to be especially aware
of any problems in setting up the lab, calibrating instruments, and
taking measurements as well as problems with the materials you are using.
For advanced labs, you may want to classify the kinds of uncertainty
you have identified. Sources of uncertainty can be classified as random-those
that cannot be predicted-or as systematic-those that are related to
personal uncertainty, procedural uncertainty, or instrumental uncertainty.
Accuracy and Precision:
Accuracy refers to the closeness of a measured value to a standard or
known value. For example, if in lab you obtain a weight measurement
of 3.2 kg for a given substance, but the actual or known weight is 10
kg, then your measurement is not accurate. In this case, your measurement
is not close to the known value.
Precision refers to the closeness of two or more measurements to each
other. Using the example above, if you weigh a given substance five
times, and get 3.2 kg each time, then your measurement is very precise.
Precision is independent of accuracy. You can be very precise but inaccurate,
as described above. You can also be accurate but imprecise.
For example, if on average, your measurements for a given substance
are close to the known value, but the measurements are far from each
other, then you have accuracy without precision.
A good analogy for understanding accuracy and precision is to imagine
a basketball player shooting baskets. If the player shoots with accuracy,
his aim will always take the ball close to or into the basket. If the
player shoots with precision, his aim will always take the ball to the
same location which may or may not be close to the basket. A good player
will be both accurate and precise by shooting the ball the same way
each time and each time making it in the basket.
4. Visualizing the data:
If your data are quantitative, it may be useful to turn the table or
spreadsheet you created into a graph. If you are going to keep your data
in a table, revise the table so that it can be presented correctly in
the report. Representing your data in the proper visual format will allow
you to identify trends and relationships among variables more easily.
For assistance with graphs or tables, follow these steps:
- Establish what types of data you have, quantitative
or qualitative (refer to the Resources page in the web version
of this document; once there, choose "Data Types").
- Determine if the data should be represented as a table
or a graph (refer to the Resources page in the web version of
this document; once there, choose "Tables vs. Graphs").
- If you decide to use a graph to represent your data, determine which
type of graph is one that
best represents your data (refer to the Resources page in the web
version of this document; once there, choose "Graph Types").
- If a table is the best format for representing your data, then modify
the table you used to collect your data so that it is labeled and
organized properly (for help in making tables, refer to the Resources
page in the web version of this document; once there, choose "Designing
Tables").
- If you need help creating a spreadsheet to make a table or graph,
refer to the Resources page in the web version of this document. Once
there, choose "Excel Tutorial".
- Remember that the purpose of your table or graph is to summarize
your findings for yourself and for others and to reveal trends in
your data.
5. Making sense of your data:
Review all your drawings, tables, graphs, and other data you collected
during your lab and summarize in a sentence or two the overall finding
for the lab. Then write a few sentences about how these findings help
to answer the questions you raised in the PreLab, question 4. If you
haven't completed the PreLab, you may want to go there now.
Summarizing your data in a sentence or two helps you to understand
the lab. It is also useful for when you write the Results section of
your lab report. Considering the questions from the PreLab will be useful
for writing your Discussion.
If your lab instructor says it is OK, ask other students in the lab
about their observations. Comparing your observations to those of other
students can be valuable as a way of furthering your learning about
the subject at hand. It is also a very common practice among scientists,
which usually leads to more ideas and more laboratory investigation.
It's OK if your findings are different. Your job is to try to figure
out why, to identify the sources of the difference. You can use this
information when explaining your findings in the Discussion section
of your lab report.
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