The Nature of Science and Experimental Design- Parts 1 & 2
Instructions: Complete the following activities prior to attending lab. TYPE YOUR ANSWERS, print it, and turn it in by the beginning of lab on Thursday. This is individual work, answers should all be independently constructed.
Part 1: Questions, hypotheses, and predictions
Scientific investigations often have their origins in direct observations of living things or in reading reports of work by other scientists. Knowledge constructed through observation, testing, and measurement is termed EMPIRICAL. Initial observations lead to questions that require additional investigation. Tentative answers to a scientific question are called hypotheses; hypotheses, in turn, naturally lead to testable predictions. Predictions follow from hypotheses and indicate what outcome will be expected in an investigation if a particular hypothesis is true. Predictions can be (but don’t have to be) formulated as if….then statements.
Scientific Questions:
Besides the need to be empirical, “good” scientific questions should possess a number of important characteristics. Some of these characteristics overlap with those required to be empirical, others allow for a reasonable attempt to be made at answering a meaningful question. Remember, a “good” scientific question leads to the formation of falsifiable hypotheses, which leads to the formation of testable predictions. Scientific questions should be testable, measurable, controllable (if experimental), definable (question is not too broad), and applicable (question has some meaning). For example, the question “How does temperature affect the growth rate of bacteria?” is a pretty good scientific question (it could be better defined with specific temperatures). Here are a few examples of bad scientific questions. For each question, determine what is wrong with the specific question and provide an improved version (or related version) of the question. An example is provided.
1. EXAMPLE: How many organisms are affected by plastics?
Not definable (too broad/vague). What proportion of seagulls observed at Newport Pier ingests plastic?
2. How much plastic is under the bench outside?
3. Is plastic found on the street worse than plastic found on the grass?
4. Why is there trash on campus?
Hypotheses & Predictions:
Hypotheses are tentative answers to scientific questions. In addition to possessing all the important characteristics of a good scientific question, and being able to lead to a testable prediction, hypotheses must also be falsifiable, meaning it is possible to disprove them. An example of a hypothesis that is not falsifiable is “The temperature in Death Valley can reach 65°C”. It is impossible to determine if there is no combination of circumstances will cause the temperature be that high, so we can never prove it wrong. A similar hypothesis can be proposed that is falsifiable “The temperature in Death Valley cannot reach 65°C”. This example is falsifiable…it would only take one record-setting hot day to refute the hypothesis and disprove it. Consider the following examples and indicate whether they meet all the characteristics of a scientific hypothesis.
To better understand these concepts, consider the following example. In many parts of the country, when daily temperatures drop in the fall and day length decreases, some trees undergo color changes in their leaves and eventually shed them. One might ask:
Q: Are seasonal changes in the environment responsible for causing leaves to drop?
To tentatively answer this question, one or more hypotheses can be developed:
H1: the decline in average daily temperature causes leaves to turn different colors & be shed
H2: the decline in day length triggers changes that cause leaves to turn colors & be shed
H3: A combination of both external factors brings about the color changes & shedding.
One prediction that emerges from these hypotheses is:
P1: (If H1 is correct…) If plants are exposed to decreasing temperatures over several weeks while keeping day length constant, then the leaves should result in color changes.
What other predictions would you make from these hypotheses?
P2: _____________________________________________________________________
P3: _____________________________________________________________________
The following examples include good and bad hypotheses. If possible and necessary, improve upon the hypothesis; then write a testable prediction for each. An example is provided.
1. EXAMPLE: All the cells in a human body possesses the same genome.
(Can’t create a testable prediction…too many cells to test all of them!)
Improved: Cells in multiple tissues from the human body possess the same genome.
Prediction: Cells sampled from the liver, skin, and intestines will have identical genomes if sequenced.
2. Whales feel emotions like love and sorrow.
3. Queen palm trees use more water per unit mass than redwoods.
4. The diversity of animal life on Earth is the result of intelligent design.
5. More pizza is consumed by college students on nights with a full moon.
Part 2: Variables
Read the material below and answer the questions that follow.
Hypotheses state a tentative relationship. In H1 above it is the relationship between daily temperatures and change in leaf color. What is it for H2 and H3?
H2: ___________________________________________________________
H3: ___________________________________________________________
For H1 the two related factors, called variables, are average daily temperature and second, leaf color change. The temperature is called the independent variable. It is the one the scientist controls or that varies on its own during the experiment. The other, color change, is a dependent variable, which according to the hypothesis is “dependent” for its expression on the independent variable. Another way of saying it is that the changes in the independent variable cause changes in the dependent variable. The idea of a “variable” comes from the fact that both temperature and color change can vary in their expression or value. Temperature can have a range of values; color change values can occur or not occur, or occur at a particular rate, or occur to different degrees (e.g. different colors). Hypotheses, then are statements about possible relationships between independent and dependent variables.
For H2 and H3, determine what the independent and dependent variables are and indicate what range of values they might take.
Hypothesis Indep. Variable Range of values Dep. Variable Range of values
H2
H3
There is a third important type of variable which can have great influence on the outcome of a scientific investigation, the nuisance variable. Nuisance variables are factors that, in addition to the independent variable, can influence the dependent variable and thus, the outcome of the study. These also may be called controlled variables or constants, because scientists try hard to control or standardize them so as not to confound their results. Consider the study of the relationship between temperature and leaf color. If in an investigation during the fall, day length shortened as temperatures dropped over several weeks, it would be difficult for the scientist to determine which factor caused the change in leaf color. For this experiment, day length is a nuisance variable and needs to be controlled in order that the results are not confounded. There are many other nuisance variables and ways of dealing with them that are dealt with in the next section.
Relationships between variables:
In Biology, many of our questions pertain to relationships between two or more variables, such as blood pressure and heart disease, or elevation and reproductive output in birds. The following are variables with potential relationships. For each pair of variables:
a. Write a question about the relationships
b. Write two or more hypotheses (possible explanations) to tentatively answer the questions
c. Indicate the independent variable and a range of values it might take
d. Indicate the dependent variable and a range of values it might take
e. Make a prediction about the outcome of a scientific investigation to test one of your hypotheses
Variables:
1. Studying and grades.
2. Bacterial growth and temperature.
3. Muscle size and muscle strength
4. Select your own two variables
This lab activity was adapted from: Gaines, M., Gaydos, B., & Griswold, J.G. (2012). Peer-Led Team Learning Introductory Biology, Module 8: Biology is a Science. Online at http://www.pltlis.org.
Originally published in Progressions: The Peer-Led Team Learning Project Newsletter, Volume 7, Number 1, Fall 2005.
