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Difficulty: MediumIdentifying Independent, Dependent, and Controlled Variables

A marine biologist conducted an experiment to investigate how varying water salinity affects the oxygen consumption rate of shore crabs (*Carcinus maenas*). Five identical tanks were prepared with water salinities of 10 ppt10\text{ ppt}, 15 ppt15\text{ ppt}, 20 ppt20\text{ ppt}, 25 ppt25\text{ ppt}, and 30 ppt30\text{ ppt}. All tanks were held at a constant water temperature of 18∘C18^\circ\text{C}, and crabs of equal body mass were used in each trial.

Match each experimental component on the left with its corresponding variable classification on the right.

  • Water salinity level (10 ppt10\text{ ppt} to 30 ppt30\text{ ppt})Independent Variable
  • Oxygen consumption rate of the crabsDependent Variable
  • Water temperature (18∘C18^\circ\text{C}) and crab body massControlled Variables

Answer

Water salinity level matches Independent Variable; Oxygen consumption rate matches Dependent Variable; Water temperature and crab body mass match Controlled Variables.
Water salinity level is systematically adjusted by the researcher across trial groups, making it the independent variable. The oxygen consumption rate is measured to quantify the crabs' metabolic response, making it the dependent variable. Parameters kept constant across all tanks—such as water temperature (18∘C18^\circ\text{C}) and crab body mass—ensure fair testing and serve as controlled variables.

Step-by-Step Solution

1
Identify the factor intentionally manipulated across treatment groups.
The researcher deliberately set five different water salinities (10 ppt10\text{ ppt} to 30 ppt30\text{ ppt}), establishing salinity as the independent variable.
The independent variable is the condition changed or controlled by the scientist to test its effects.
2
Identify the parameter measured as an outcome of the experiment.
The oxygen consumption rate of the crabs was recorded to evaluate physiological response, establishing it as the dependent variable.
The dependent variable represents the response measured to observe the effect of changing the independent variable.
3
Identify environmental and biological factors held uniform across all trials.
Water temperature (18∘C18^\circ\text{C}) and crab body mass were kept strictly constant, classifying them as controlled variables.
Controlled variables must be standardized so confounding variables do not skew the relationship between the independent and dependent variables.

Key Concept

Experimental variables are categorized into independent variables (manipulated inputs), dependent variables (measured outcomes), and controlled variables (standardized parameters).
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Topics

Identifying Independent, Dependent, and Controlled Variables

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