When performing an experiment, the researcher needs to define the variables: dependent -fishes response-, independent -the ice cubes' presence/absence -, and controlled -the jar and environmental conditions- variables.
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Before performing any experiment, the researcher - Mr. Edison- needs to define the variables.
• Independent variable: Refers to all the variables in an experiment that provoke a response in another variable. An independent variable is the one that changes or is modified to analyze its effects on another variable. The researcher changes on purpose the independent variable to observe the response of the dependent variable.
In the exposed example, the independent variable is the presence/absence of ice cubes.
• Dependent variable: Refers to the variable, which response depends on any change in the independent variable. It represents a quantity of something which value depends on how the independent variable is modified. The change in the dependent variable might be proportional or inversely proportional to the change in the manipulated variable.
In the exposed example, the dependent variable is the fishes survival or the fishes death.
• Controlled variables are kept constant. Some variables might influence the results, so they need to remain constant to cause no effect on the dependent variable response. They need to be controlled. Controlled variables do not influence the results.
In the exposed example, the controlled variable is the fishes species, the size of the jar, and other environmental conditions such as food, light exposure, water oxygen saturation, pH, temperature, etcetera.
Knowing this, we can answer the questions.
Q1. Why did Mr. Edison buy the same kind of fish as those that had died? *
A1. To make sure that his fish would have the same response behavior as the fish that died.
The kind of fish is a controlled variable. To perform the experiment properly he needs to use the same species because another species might behave differently to the ice cubes presence.
Q2. Why did he keep the fish in the aquarium he bought for two days before started his investigation?
A2. To ascertain that they can survive in their new environment.
Probably the conditions in the aquarium were different from the conditions in Edison's house. So before performing the experiment, he needed to be sure that the animals got to adapt to their new environmental conditions -which are also controlled variables-. In this way, he could be sure that the switch of the environment -from the aquarium to his house- did not influence the fishes' response to ice.
Q3. Why did he use identical jars in his investigation?
A3. It was necessary for all conditions to be identical except one: the experimental variable.
Jars size are also a controlled variable. Different sizes would mean different amounts of water, space, etcetera. He needed to ensure that these variables were kept the same and had the same influence on fish. Otherwise, they could be influencing the animals' survival.
Q4. Why did he put ice cubes in Jar No. 1?
A4. To find out if there would be a difference in the behavior of the fish in the two jars.
The presence/absence of ice cubes was the treatment, the independent variable. He needed to know if fishes died because of the ice cubes, so he compared how animals behave in the presence and the absence of ice cubes. To make a proper comparison, he exposed all the animals to the same environmental conditions excepting for the ice cubes exposition. He put ice on only one jar and observed the survival of animals in both jars.
Q5. What did Edison want to find out from this experiment?
A5. The reason for the death of fishes in the aquarium
He wanted to know if his fished died because of the ice cubes or if there was any other reason that provoked their death. If ice was not the cause of death, then animals in both jars should have survived. If ice was the cause of death, then only the animals in Jar number 1 should die.
By defining the variables involved in the experiment, Mr. Edison could perform a simple experiment and got to know if ice cubes caused the fishes' death.
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