Problem: Read the following information and determine the type of study:
Researchers want to compare the effect of 3 fluoride treatments per day versus 2 fluoride treatments per day on existing dental caries. Sixteen participants wore a partial prosthetic enamel and dentine specimen, with a pre-formed dental lesion on it. Participants were randomly assigned to two groups, using a double-blind method. One group received the fluoride rinse twice per day and a placebo once per day, and the other got fluoride three times per day. At the end of the experiment, specimens were retrieved for fluoride analysis and the assessment of integrated mineral loss. (Laheij, van Strijp, van Loveren, 2010)
A. One-shot case study design
B. Counterbalanced design
C. Posttest-only control group design
D. One-group pretest-posttest design
E. Nonequivalent control group design