Astudent researcher conducts an experiment that investigates the effects of
a new mathematics website on student achievement. she randomly assigns
students to two groups: one that receives supplemental instruction from the
website and one that doesn't receive the supplemental instruction. each
sample consists of 20 students. observations of student engagement are
rated by the researcher, according to an engagement scale. how could this
study be improved?
select all that apply.

a. increase the sample size.

b. have another researcher provide ratings, as well.

c. limit observations to students who are engaged.

d. administer a quiz or exam that gauges student understanding.