Final answer:
The researcher's method violates the principle of random assignment. Random assignment is vital for reducing systematic bias and ensuring that any observed differences are due to the treatment administered, not pre-existing differences. Proper design also prevents experimenter bias by keeping the test scorer blind to group assignments.
Step-by-step explanation:
If a researcher created two groups by assigning the first 30 people to the experimental group and the last 30 to the control group, the process would violate the principle of random assignment. Random assignment is a cornerstone of a sound experimental design, ensuring that each participant has an equal chance of being assigned to either group. This method is crucial to minimize systematic differences between the experimental and control groups and to prevent experimenter bias.
In an ideal experimental scenario, aside from the application of the independent variable, there should be no differences between the groups. Random assignment mitigates the risk of confounding variables and ensures that observed differences after the experiment can be confidently attributed to the manipulation of the independent variable. Statistical software is often employed to randomly assign participants to groups, thereby adhering to ethical experimental practices and preserving the validity of the study.
Conversely, systematically assigning participants to groups, as in the scenario described, increases the likelihood of systematic bias. Pre-existing group differences that are not addressed may confound the results, making it difficult to determine if outcomes are due to the treatment or other factors. Additionally, the person scoring the tests should ideally be blind to group assignments to prevent experimenter bias, which could influence the interpretation of results.