The _______________________ receives the experimental manipulation, whereas the ________________________ group does not. Researchers must also ______________________________ define variables of interest. It is best practice to avoid ______________________ bias, or the possibility that a researcher's belief's will skew the results of the study. Using _____________________-blind (participants are unaware of their group assignment) or ___________________________-blind (both participants and researchers are unaware of participant group assignment) procedures can help reduce this bias.