Exercise 1: Introduction, Scientific Method & Measurements I
______________________________________________________________________________
OBJECTIVES:
1. Familiarize yourself with lab policies and practices
2. Learn and practice the scientific method
3. Learn about accuracy vs. precision
4. Learn about the use of basic statistics
_____________________________________________________________________________
INTRODUCTION:
Science is a way of acquiring new knowledge to understand the world around us.
Scientists approach all natural phenomena using standardized deductive reasoning, more
commonly referred to as the scientific method. The scientific method may be expressed as a
series of steps: 1) making observations, 2) creating questions, 3) forming hypotheses, 4)
conducting empirical tests, 5) quantifying data, 6) testing hypotheses, and 7) drawing
conclusions (Fig. 1), each of which will be discussed in further detail and implemented
throughout the semester.
In this course, you will learn and apply the underlying principles and techniques used by
scientists to gather data of basic biological processes. You will also gain an overall
understanding of how this knowledge is used in areas ranging from the development of
treatments for diseases to the effects of global warming on our planet. In addition, this lab will
introduce you to basic techniques required to perform experiments later on in the semester.
______________________________________________________________________________
A GENERAL NOTE ON PERFORMING TASKS IN THIS COURSE:
In any experiment, wait for your TA to describe or demonstrate the proper procedure before
starting. Never perform an experiment unless you are sure about what you are doing. If you
have any questions, it is much better to ask than to proceed incorrectly and fail or cause danger
to yourself or others.
______________________________________________________________________________
Task 1 - SAFETY: Learn about safety in a biology laboratory.
1. Review the lab safety handout with your instructor.
2. Note the location of each of the following safety equipment/items
a. Shower ___________________________________________________
b. Eyewash___________________________________________________
c. Medical emergency procedure__________________________________
d. Fire extinguisher_____________________________________________
e. Emergency gas cut-off valve___________________________________
f. Phone_____________________________________________________
1
Task 2 - THE SCIENTIFIC METHOD
The scientific method is a way of posing questions about the natural world that can be
tested through repeated and unbiased experiments. It is a highly effective process for solving
problems and answering questions. Generally, the scientific method includes the following steps:
Figure 1. The Scientific Method
The scientific method begins with a question about a particular phenomenon; for
instance, the role of smoking in lung cancer. After conducting background research and learning
more about the subject of interest, a scientist formulates a possible explanation for the
phenomenon i.e., a hypothesis. Hypotheses are always phrased as statements, not questions.
In formulating a hypothesis, assumptions are stated and a tentative explanation is proposed. In
the lung cancer example above, the general hypothesis is that smoking causes lung cancer. It is
important that hypotheses are falsifiable. In other words, based on the results of the data
collected, a hypothesis is either retained or rejected. The results can never “prove” the
hypothesis, but can lead to the conclusion that it cannot be rejected.
To ensure that results obtained are not erroneous (such as false positives or false
negatives) due to random chance, we need to consider alternative explanations. In the case of
2
our lung cancer example, an alternative explanation would be that lung cancer is caused by
something else, not smoking (i.e. smoking does not cause lung cancer). In lieu of this, scientists
formulate two types of hypotheses: 1) a null hypothesis (Ho), and 2) an alternative hypothesis
(Ha). Note that Ho and Ha are always paired. Ho states that any relationship found between
treatments is due to random chance, while Ha predicts that a relationship actually exists (i.e., the
exact opposite of Ho). In essence, Ho is a statement that we are trying to nullify/falsify in favor of
the Ha. In the lung cancer example,
Ho: Smoking does not cause lung cancer
Ha: Smoking causes lung cancer
In order to test the hypotheses generated, an experiment needs to be designed. A plausible
experiment to test Ho and Ha above might be to track groups of smokers and non-smokers and
compare the occurrence of lung cancer in each group. Predictions about the experimental
outcomes are then made using if-then statements; if the data supports Ho, then we should not
observe a higher incidence of lung cancer in smokers, compared to non-smokers. In other words,
if no observable differences between smokers and non-smokers are noted, then we fail to reject
Ho.
Whether a hypothesis can be tested is dependent on how well an experiment is designed.
A good experiment should have all variables identified and proper controls to limit extraneous
influences. A variable is anything that is controlled, manipulated or measured. An independent
(experimental) variable is the one that is manipulated during the experiment while the
dependent (response) variable changes in response to the independent variable. Controls are a
crucial component of every experiment and are kept constant throughout the experiment in order
to examine the impact of the independent variable. In the lung cancer example, smoking is the
independent variable while the occurrence of lung cancer would be the dependent variable. The
control in this case would be a group of non-smokers.
Using the Scientific Method
A. Description of the Problem
You will work through this exercise in your groups, periodically coming together as a
class to explain your reasoning and to decide on how to proceed with the remainder of the task.
Your assignment is to develop a scientific hypothesis and test it. The topic will be neuromuscular
reaction time. This can be easily determined by measuring how quickly a person can grasp a
falling meter stick and will be recorded as millimeters (mm) free fall.
Procedure:
The person whose reaction time is being measured (the
subject) sits at a table with her or his forearm on the top and the
hand extended over the edge, palm to the side and the thumb and
forefinger partially extended (Fig. 2). A second person holds a
meter stick just above the extended fingers and drops it. The
subject tries to catch it. The distance the meter stick drops before
being caught is a measure of the subject’s reaction time.
3
Figure 2. Measuring reaction time
Your assignment is to generate a scientifically testable hypothesis regarding reaction time
in individuals with different characteristics. Remember that a null hypothesis (Ho) is always
paired with an alternative hypothesis (Ha). You will then design an experiment to test the
hypothesis, collect and analyze the data, and decide whether to reject or fail to reject your Ho.
For example, you might investigate if there are differences in reaction times between those who
play musical instruments and those who do not, if there are gender differences, or if differences
exist between right and left-handed individuals. The design will depend on the hypothesis that
you decide to test as a class.
B. Summarizing Observations
1. As a group, begin your discussion of this assignment by summarizing all possible
questions to answer. Are neuromuscular responses the same for all people or might they
vary by athletic history, gender, body size, age, hobbies requiring manual dexterity, left
versus right hand, or other factors? For example, you might expect differences in the
physiological responses of those who exercise. What other factors might influence the
response time?
C. Asking Questions
2. Research starts by asking questions which are then refined into hypotheses. Review the
group observations that you listed above and write down scientifically answerable
questions that your group has about reaction time in people with different characteristics.
Present your group’s best question to the class.
D. Forming Hypotheses
3. As a class, review the questions posed by all groups. Examine the questions for their
answerability. As a class, you will now vote on the best question that all groups will
examine. Write that question in the space provided below. In addition to the question
decided on by the class, you will determine if the size of a person’s hand affects reaction
speed (distance from beginning of the palm to tip of the middle finger).
4
4. In your group, state the question above as a prediction. For example, because piano
players constantly train their neuromuscular reaction time, you might expect that they
would have short reaction times. Use this prediction as a basis for forming a testable
hypothesis. Continuing with the example, you might propose for a hypothesis that there
would be no significant difference between piano players and non-musicians in reaction
time. The alternate hypothesis would be that there is a significant difference between
musicians and non-musicians. Remember that hypotheses must be testable through
experimentation or further data gathering. State Ho and Ha below.
a. Describe what makes your Ho testable.
b. State the hypothesis testing if the size of one’s hand affects reaction speed:
E. Designing an Experiment
To test a hypothesis, a controlled experiment must be devised. It should be designed to collect
evidence that would prove the hypothesis false. Discuss the design of the experiment to be
performed including the variables below.
5. What are the variables in your experiment?
a. Which of the variables is (are) the independent variable(s)?
b. Which of the variables is (are) the dependent variable(s)?
c. What variables will be controlled and how will they be controlled?
5
6. Having decided which variables fit into these categories, you must now decide on a level
of treatment and how it will be administered. How will you standardize measurements
across groups in the lab so that the results are comparable?
7. Recognizing that the subject may anticipate the dropping of the meter stick or be
momentarily distracted when it is dropped, how many times will you repeat the
experiment to have confidence in your results?
8. Present your group design to the class. Consider the experimental designs proposed by
other groups. As a class, vote on an experiment that best tests the hypothesis. Note the
experimental design below. Make sure that each person in the group serves as a
subject.
F. Procedure
9. As a class, write a protocol describing about how the experiment will be performed. All
groups will follow these procedures. Also, design a table in which you will record your
data. Put the procedures and data table in the space provided.
6
10. Perform the experiment as a group.
G. Data Recording
11. Combine your group results with those from the other groups to generate a class dataset.
Make sure to copy all of the data for analysis. Record all the data in the space below.
7
H. Data Summarization
12. Graph your data to investigate the relationship between your independent variable and
reaction speed for each experiment. Divide the space below into two graphs to display
the data for each hypothesis (hand size and variable determined by the class). Note: The
independent variable is plotted on the horizontal (x) axis while the dependent variable is
plotted on the vertical (y) axis.
8
I. Data Interpretation
13. Write a few sentences that summarize the trends that you see in the data.
J. Conclusion
14. Return to the hypotheses (Ho and Ha) that were formulated at the beginning of the
experiment. Compare them to the experimental results. Do you reject or fail to reject the
hypothesis? Why? Cite the data used in making the decision.
K. Discussion
15. As you conducted this experiment and analyzed the results, additional questions probably
came into your mind. As a result of this thinking and the results of this experiment, what
do you think would be an interesting hypothesis to test if another experiment were to be
done?
16. Evaluate the design of your original experiment. Be as critical as you can. Were any
variables not controlled that should have been? Is there any source of error that you now
see but did not before? Note: In any experiment there is “error”. Error is the difference
between a measured value of quantity and its true value. It is not a "mistake". Variability
is an inherent part of things being measured.
9
______________________________________________________________________________
Task 3 – DATA COLLECTION: Accuracy vs. Precision
When taking scientific measurements both accuracy and precision need to be considered.
Accuracy is the exactness of a measure whereas precision is the ability of an instrument to
repeatedly provide a measure. In the example below (Fig. 3), arrows that land closer to the bull’s
eye are considered more accurate than those landing further away, while arrows that cluster
tightly together in a particular area of the target are more precise than those scattered about.
Figure 3. Precision is the ability to consistently hit the same area of the target, while
accuracy is how close you come to the center of the target.
In the lab, scientists use a balance/scale to measure mass in grams (g), however the
number of decimal places reported varies from scale to scale. The scale’s accuracy is determined
by how close its measurement of an object’s mass is to the object’s actual value. If greater
accuracy is required, a scale that provides further definition should be used, i.e., one that reports
a greater number of decimal places. In contrast, when referring to precision, we are talking about
how well the scale performs, i.e., its reproducibility or ability to consistently give similar mass
measurements every time the same object is weighed.
Procedure:
1. Your TA will demonstrate how to correctly use the tools for measuring volume that are at
your station.
2. Use the 5mL pipette and 100mL beaker (filled with about 25mL of water) at your table to
practice using the pipette.
10
3. Your TA will now demonstrate how to use the balance. Make sure you learn and
understand how to “zero” the balance. Note: The process of subtracting the mass of the
device holding a substance from the mass of the substance plus the holding device is
called taring. When you zero the scale, you are taring.
4. Weigh an empty, 100mL beaker.
Mass of beaker alone = __________.
5. Using a graduated cylinder, fill the beaker with 25mL of water and weigh it again.
Mass of beaker + water = ________.
Note: When reading the amount of liquid within a container, you must consider that
liquids tend to adhere to the sides of the vessel where they are. When liquid does this, it
forms a meniscus (Fig. 4). Readings should be taken at the bottom of the meniscus.
Meniscus
Figure 4. Meniscus of a liquid-filled graduated cylinder
6. Subtract the mass of the beaker alone from the mass of the beaker with water for the mass
of the water.
Mass of water = ________.
7. Empty the beaker again and use a pipette to add the following volumes of water in
milliliters to your beaker: 5.0, 4.5, 4.3, 4.2, 4.0, 3.5, 3.3, 3.2, 3.0, 2.5, 1.5, 1.0 (this should
equal a total of 40mL). We will now check the accuracy of the pipette by weighing the
water and using the density of water to calculate volume. Density = mass per unit
volume (mass/volume). The density of water at room temperature = 0.998g/ml.
8. Weigh the beaker with the water added.
Mass of beaker + water = _________.
11
9. Subtract the mass of the empty beaker (you measured this previously) from the mass of
the beaker with the water.
Mass of water = _________.
10. Divide the mass of the water in the beaker by the density of water to get the volume of
water in the beaker.
Volume = __________.
Question:
Is the volume more or less than 40mL? If it is not the same, what can we say about the
precision and accuracy of the pipette?
11. Calculate the experimental error using the following formula:
% error = [(actual value – theoretical value)/ theoretical value] x 100
% error = ________________
12. What does a positive error value mean? What about a negative value? What are some
sources of possible error?
13. If you did the same experiment but instead you added two 20mL portions, do you think
the error would change? Why?
12
Because there is usually some degree of error introduced when we take measurements,
scientists take multiple readings and then use statistics to get a more accurate representation of
the true measurement to evaluate how accurate the sampling was. Some frequently used
statistical measures include mean (arithmetic average of a group of measurements), median
(middle value of a group of measurements), and mode (most frequently occurring number in a
group of measurements). In addition, range, variance and standard deviation can be used to
measure variability.
Range = absolute difference between lowest and highest value in the data set
Variance = [sum (measured value for each sample – mean)2] / (sample size -1)
= ∑ (x-x)2
N-1
Standard Deviation = √variance
14. Using the values in the Table 1, perform all calculations necessary to obtain the variance
and standard deviation for the example data set.
Table 1:
Measure Collected
15
20
25
16
20
24
10
20
30
Sum (∑) =
Sample Size (N) =
Mean =
Measure - Mean
(Measure – Mean) 2
∑=
N – 1=
Variance =
Standard Deviation =
15. We will now apply what you have learned about the use of general statistics towards
examining the data you collected using the pipettes. Each group will select one person to
go to the board to write down the value you calculated for water volume using density.
16. Copy the data on the board in Table 2 and compute all listed variables.
13
Table 2:
Group Water vol. (ml)
1
2
3
4
5
6
∑=
Range =
N=
Mean =
Measured vol. – Mean vol.
(Measured vol. – Mean vol.) 2
∑=
∑/ (N -1) =
√∑/ (N -1) =
17. Do the values between each group differ? If so, is any value very different from the
others? If yes, check these for possible errors. If you can determine there was a gross
error, you can then feel comfortable about eliminating that particular data point from your
data set. Look for outliers and check them to make sure they are not legitimate measures.
Sometimes outliers are true measures, but sometimes they are not. If determined not to be
a true measurement, those data points should be discarded. However, if the outliers are
true measures, then we should keep them. If your group does reject any data, write down
the reasons for rejection in the space provided below.
18. Calculate the mean for water volume and record this value in Table 2.
Questions:
a. Is this value closer to the theoretical value of 40mL than the individual measurements?
Why?
b. Is the range a good measure of the variability of the data? If you used proper lab
technique, should the range be small or large? Why?
14