To test the effect of a new exercise routine on muscle growth, a researcher sets up the following experiment: 60
volunteers subjects (40 men and 20 women) of average
build are selected to participate in the study. Their body
mass and muscle sizes (biceps, thighs, etc.) are
measured. The researcher gives all the men a DVD with
the new exercise routine, and the women a DVD with a
standard routine, but only the researcher knows this.
The results of the experiment are likely to be invalid mostly
because
the subjects did not know if they were getting the
real treatment.
the gender of the patient is a confounding variable
in this experiment
the treatment group and control group were not the
same size.
the subjects were volunteers.
None of the above