Through control, the researcher can reduce the influence of extraneous variables. variables exist in all studies and can interfere with obtaining a clear understanding of the relationships among the study variables. For example, if a study focused on the effect of relaxation therapy on the perception of incisional pain, the researchers would have to control the extraneous variables, such as type of surgical incision and time, amount, and type of pain medication administered after surgery, to prevent their influence on the patient’s perception of pain. Selecting only patients with abdominal incisions who are hospitalized and receiving only one type of pain medication intravenously after surgery would control some of these extraneous variables. Controlling extraneous variables enables researchers to determine the effects of an intervention or treatment on study outcomes more accurately.
The levels of evidence from strongest to weakest are as follows :
• Systemic review of experimental studies (well-designed randomized controlled trials [RCTs])• Meta-analyses of experimental (RCT) & quasi-experimental studies• Integrative reviews of experimental (RTC) & quasi-experimental studies• Single experimental study (RCT)• Single quasi-experimental study• Meta-analysis of correlational studies• Integrative reviews of correlational & descriptive studies• Qualitative research meta- synthesis & meta-summaries• Single correlational study• Single qualitative or descriptive study• Opinions of respected authorities based upon clinical evidence, reports of expert committees (Grove, Gray, & Burns, 2015, p. 24 )
Systemic review is reviewed by experts in the field of research who meticulously evaluate the content. Meta-analysis uses quantitative approaches to review the research from multiple studies. Integrate review is a compiling of research that has been revised and abridged. Single experimental study is a true experimental study where researchers are typically unaware as to which participants are receiving the intervention. Meta-analysis compiles qualitative data while searching for common themes among comparable research studies (Peterson et al., 2014, p. 62-63)