Question

part c: experimental design
an experiment involves defining variables, outlining a procedure, and determining controls to be used as the experiment is performed.
determining variables: there are different kinds of variables in science
the independent variable (experimental variable) - the factor the investigator varies during the experiment (or has a natural variation in the case of an observational experiment). the independent variable produces an effect.
the dependent variable - what the investigator measures (or counts or records). it is what the scientist hypothesizes will be affected during the experiment. a way to remember this is that the dependent variable depends on the independent variable. for example, the incidence rate of lung cancer (dependent variable) depends on exposure to cigarette smoke (independent variable).
controlled variables - those variables held constant during an experiment, so their effect on both groups being compared is equal. the control of these variables is important for both observational and manipulative studies.
exercise 6
you hypothesize that temperature will determine the growth rate of ferns. you grow the plants in small pots and place some in an incubator (40°c), others in a laboratory room (20°c), and others in a refrigerator (10°c).
a) what is the independent (experimental) variable?
b) what is the dependent variable and how could this be measured?
c) what are examples of controlled variables?
d) what precautions might you take to insure that these really are controlled (the same for every treatment)?
exercise 7: choosing a procedure
after formulating a hypothesis and selecting the independent and dependent variables, the investigator must find a method or procedure to conduct the experiment. the process of outlining the procedure includes determining levels of treatments, deciding on number of replicates, and determining the control treatment if applicable.
levels of treatment: early in the procedure, the scientist must determine appropriate values to use for the independent variable. the values are called levels of treatment. this judgment is usually based on prior knowledge of the system. for example, in the study of the effect of temperature on weight gain in bluegill sunfish, the scientist should have enough knowledge of the physiology of bluegills to use appropriate temperatures. subjecting animals to extremely high or low temperatures could kill them, and no useful data would be obtained. similarly, a scientist wanting to see if fertilizer additions would stimulate corn production in fields of a particular soil type would need to know something about amounts typically used and vary the treatments around those levels.
replication: another essential aspect of experimental design is replication. this means that the scientist applies the different levels of the independent variable to more than just one experimental unit. an experimental unit could be an individual organism, a test tube culture of millions of bacteria, or an entire farm field. for example, in the fern experiment, you would have more than one plant at each temperature.

Explanation:

Step1: Identify independent variable

The factor being varied is temperature, so the independent variable is temperature.

Step2: Identify dependent variable and measurement

The growth rate of ferns is what is being measured. It could be measured by the increase in height of the ferns over a set period, or the number of new fronds produced. So the dependent variable is the growth rate of ferns, and it can be measured by height increase or number of new fronds.

Step3: Identify controlled variables

Controlled variables could include the type of soil, the size of the pots, the amount of light, the type of fern (species), and the amount of water. These should be kept the same for all plants in different temperature - conditions.

Step4: Precautions for controlled variables

Use identical pots and soil for all plants. Place them in areas with the same light intensity (if light is a controlled variable). Use ferns of the same species and similar initial size. Water them with the same amount of water at the same time intervals.

Answer:

a) Temperature
b) Dependent variable: Growth rate of ferns. Measurement: Increase in height or number of new fronds.
c) Type of soil, size of pots, amount of light, type of fern (species), amount of water.
d) Use identical pots and soil, same light intensity, ferns of same species and similar size, water with same amount at same time - intervals.