An experimental study is one in which there is a study group and a control group as well as an independent (causal) variable and a dependent (outcome) variable. Subjects who participate in the study are assigned randomly to either the study or control conditions. The investigator manipulates the independent variable and observes its influence upon the dependent variable. This study design is similar to those that the reader may have heard about in a psychology course. Experimental designs also are related to clinical trials, which were described earlier in this chapter.
Experimental studies are used extensively in product quality control. The manufacturing and agricultural industries have pioneered the application of statistical design methods to the production of first-rate, competitive products. These methods also are used for continuous process improvement. The following statistical methods have been the key tools in this success:
• Design of Experiments (DOE, methods for varying conditions to look at the effects of certain variables on the output)
• Response Surface Methodology (RSM, methods for changing the experimental conditions to move quickly toward optimal experimental conditions)
• Statistical Process Control (SPC, procedures that involve the plotting of data over time to track performance and identify changes that indicate possible problems)
• Evolutionary Operation (EVOP, methods to adjust processes to reach optimal conditions as processes change or evolve over time)
Data from such experiments are often analyzed using linear or nonlinear statistical models. The simplest of these models (simple linear regression and the one-way analysis of variance) are covered in Chapters 12 and 13, respectively, of this text. However, we do not cover the more general models, nor do we cover the methods of experimental design and quality control. Good references for DOE are Montgomery (1997) and Wu and Hamada (2000). Montgomery (1997) also covers EVOP. Myers and Montgomery (1995) is a good source for information on RSM. Ryan (1989) and Vardeman and Jobe (1999) are good sources for SPC and other quality assurance methods.
In the mid-1920s, quality control methods in the United States began with the work of Shewhart at Bell Laboratories and continued through the 1960s. In general, the concept of quality control involves a method for maximizing the quality of goods produced or a manufacturing process. Quality control entails planning, ongoing inspections, and taking corrective actions, if necessary, to maintain high standards. This methodology is applicable to many settings that need to maintain high operating standards. For example, the U.S. space program depends on highly redundant systems that use the best concepts from the field of reliability, an aspect of quality control.
Somehow, the U.S. manufacturing industry in the 1970s lost its knowledge of quality controls. The Japanese learned these ideas from Ed Deming and others and quickly surpassed the U.S. in quality production, especially in the automobile industry in the late 1980s. Recently, by incorporating DOE and SPC methods, US manufacturing has made a comeback. Many companies have made dramatic improvements in their production processes through a formalized training program called Six Sigma. A detailed picture of all these quality control methods can be found in Juran and Godfrey (1999).
Quality control is important in engineering and manufacturing, but why would a student in the health sciences be interested in it? One answer comes from the grow ing medical device industry. Companies now produce catheters that can be used for ablation of arrhythmias and diagnosis of heart ailments and also experimentally for injection of drugs to improve the cardiovascular system of a patient. Firms also produce stents for angioplasty, implantable pacemakers to correct bradycardia (slow heart rate that causes fatigue and can lead to fainting), and implantable defibrillators that can prevent ventricular fibrillation, which can lead to sudden death. These devices already have had a big impact on improving and prolonging life. Their use and value to the health care industry will continue to grow.
Because these medical devices can be critical to the lives of patients, their safety and effectiveness must be demonstrated to regulatory bodies. In the United States, the governing regulatory body is the FDA. Profitable marketing of a device generally occurs after a company has conducted a successful clinical trial of the device. These devices must be reliable; quality control procedures are necessary to ensure that the manufacturing process continues to work properly.
Similar arguments can be made for the control of processes at pharmaceutical plants, which produce prescription drugs that are important for maintaining the health of patients under treatment. Tablets, serums, and other drug regimens must be of consistently high quality and contain the correct dose as described on the label.
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