Answer
The conclusion is not valid.
Work Step by Step
The participants voluntarily joined the experiment and they are not controlled by the researcher. Thus, the results may be confounded with other variables.
Introductory Statistics 9th Edition
by Mann, Prem S.
Published by
Wiley
ISBN 10:
1-11905-571-7
ISBN 13:
978-1-11905-571-6
Chapter 1 - Section 1.6 - Design of Experiments - Exercises - Page 22: 1.35
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