Plan and evaluate a controlled investigation by identifying the independent, dependent, and controlled variables and the control group, as embedded across the STAAR Biology reporting categories (TEKS Biology scientific and engineering practices; cause and effect).

What this topic is asking

The Biology TEKS embed the scientific and engineering practices across every reporting category, and one of the most tested is planning and evaluating a controlled investigation. For STAAR Biology you need to identify the independent, dependent, and controlled variables and the control group, and judge whether an experiment is fair. This is a cause-and-effect skill, and it appears inside questions from any content area, often as a described investigation.

The variables

A simple way to keep them straight:

• Independent = what I change (I for independent).
• Dependent = what depends on it, the result I measure.
• Controlled = what I keep constant so it does not interfere.

For example, testing how temperature affects an enzyme's activity: temperature is the independent variable, the rate of reaction is the dependent variable, and the amount of enzyme, amount of substrate, and pH are controlled variables.

The control group

Without a control group, you cannot be sure a change was caused by the treatment rather than by something else. In a fertilizer test, the control group gets no fertilizer; if the fertilized plants grow taller than the unfertilized control, the difference can be linked to the fertilizer.

What makes a test fair

A fair test changes only the independent variable and keeps everything else constant. If two things change at once, you cannot tell which one caused the result. So a well-designed investigation:

1. Changes one independent variable;
2. Measures one dependent variable;
3. Keeps all other variables constant (controlled);
4. Includes a control group for comparison;
5. Where possible, repeats trials to make the result reliable.

Evaluating a flawed experiment, spotting a variable that was not controlled, or a missing control group, is a common STAAR task. The recurring theme is cause and effect: a fair test is what lets you claim that the independent variable caused the change in the dependent variable.

Try this

Q1. Define the independent variable and the dependent variable. [2]

• Cue. The independent variable is what the experimenter changes; the dependent variable is what is measured (the result).

Q2. Explain why an experiment needs a control group. [2]

• Cue. It provides a baseline with no treatment, so the experimenter can tell whether the treatment (not something else) caused the difference.