A Gentle Introduction to Statistics Using SAS Studio. Ron Cody
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is taking a drug in pill form. For example, the placebo group can be given an inert substance in a capsule. However, what about a surgical technique or acupuncture? How can the subject be blinded in that case? One study, done in the early 1960s, was designed to test if freezing the stomach with a balloon filled with liquid nitrogen would cure stomach ulcers. One group of subjects had their stomachs frozen. For the placebo group, the liquid nitrogen was introduced into the endoscopy tube, but then shunted out before it reached the stomach. Therefore, it looked like the real thing to the subject and to the staff (except for the person who was switching the shunt on or off). Before this study was conducted, stomach freezing was believed to help cure ulcers. However, the results of the study showed it did not work at all.
The placebo effect can be so strong that even different types of placebos can produce different results. For example, placebo injections, capsules, or pills, can produce different results. Even the color of the pill can sometimes make a difference.
Cohort Studies
A cohort is a group of people who share a common characteristic, such as taking vitamins or getting annual physical exams. A great deal of knowledge can be gained by comparing groups of subjects who differ in their lifestyles or other factors. For example, you could look at people who exercise regularly versus people who are more sedentary and compare endpoints such as blood pressure, cholesterol, or death from cardio-vascular disease. The advantage of studying cohorts is that you can compare groups containing thousands of subjects.
Farmington, Massachusetts is the site of one of most famous series of cohort studies ever conducted. This town was deemed to be a good representation of the US population. All types of surveys were (and still are) administered to people in the town who were willing to participate in the study. Physical exams and blood tests were also performed on a regular basis. Studies such as these gave epidemiologists data to determine how risk factors such as high cholesterol or high blood pressure are related to heart disease.
However, there are several problems with cohort studies. Because the groups are not randomly assigned to a treatment, the groups may differ on many other factors besides the one being studied. One of the most famous cohort studies that demonstrates how bias can affect the results compared women who took hormone replacement therapy (HRT) with women who did not. The results showed that women who took HRT were significantly less likely to experience a heart attack or stroke.
Years later, a large, double-blind, placebo-controlled clinical trial was conducted to compare HRT to a placebo. The results showed that HRT increased the likelihood of heart attacks and strokes. Why were the results of the earlier studies and the new study so different? Who were the women in the early study who received HRT? Most likely, they were more affluent, had better medical care, and might have lived a healthier lifestyle. It was because of those reasons that it looked like HRT was beneficial when it was not. This demonstrates that while cohort studies can be very useful, the results need to be looked at with more skepticism than results from clinical trials.
Case-Control Studies
The last study design to be described in this chapter is a case-control study. Cases are subjects who already have a disease or condition of interest. Controls are subjects chosen to be similar to the cases except for the fact that they do not have the disease or condition in question. Typical case-control studies compare the two groups to see whether there are differences in lifestyle, exposure to chemicals (such as cigarette smoke), or medications. One such study compared women who gave birth to babies with a condition called neural tube defects. This condition causes an abnormal brain, spine, or spinal column. Controls were women who gave birth to babies without this condition. The two groups were compared on many factors. One factor that showed a significant difference was between women who took folic acid supplements and those who did not.
Why not do a clinical trial to test if folic acid is useful in preventing neural tube defects? There are several reasons why a clinical trial would be difficult. First, neural tube defects are quite rare, so you would need to recruit thousands of women for the study. Another problem is an ethical one. If doctors believe that folic acid helps prevent this condition, how can they, in good conscience, deny folic acid to a placebo group?
Case-control studies are very useful when you are studying a rare condition. However, the results from a case-control study have to be looked at with caution. In the neural tube defect study, women who gave birth to babies with this condition may have had more vivid recollections of what they ate and did while pregnant, compared to women who had normal babies. This problem is called recall bias, and it is difficult to control for. Finally, because you start with people with the disease in one group and healthy people in the other group, you cannot compute incidence rates (how many people get the disease or develop the condition in a certain period of time).
Conclusion
The three study designs described in this chapter are all useful in comparing drugs or treatments in groups of people. There are times when you cannot blind the treatment. One example of this is if you are comparing two different methods to teach students to read. Some of the children (depending on age) might realize they are being taught differently than their friends. The results of studies that are not blinded can still provide useful information.
Chapter 3: What Is SAS University Edition?
How to Download SAS University Edition
Introduction
Many of you will be accessing SAS Studio along with SAS University Edition. If you are using SAS Studio with an edition of SAS that is not SAS University Edition, you can skip this chapter and continue on to Chapter 4, which discusses SAS Studio tasks. If you are already familiar with SAS Studio, you can jump all the way to Chapter 6 to see how you can use SAS Studio to manage and report your data and create graphs and reports. Later chapters show you how to perform most of the statistical tasks performed by statisticians. The examples in this book all use SAS University Edition, but users of SAS Studio in other environments will see that the statistical tasks are identical to the ones described here.
SAS University Edition is a full version of SAS software that is free to anyone, and it runs on Microsoft Windows computers as well as Apple laptops and Linux workstations. How can this be?
When you download this free software, you agree that you will not use it for commercial purposes. As a student or researcher using SAS University Edition to learn how SAS works, this is not a problem. Once you have mastered the statistical and other tasks using SAS University Edition, you can use those same skills with the licensed versions of SAS used by universities and in the corporate world.
A huge advantage of using SAS University Edition is that you access SAS using an interface called SAS Studio. This provides a programming environment that enables you to write SAS programs, but, more importantly, provides you with an interactive, point-and-click interface where you can quickly and easily run a large variety of statistical tests.
This looks too good to be true. Well, there is a slight complication that results from allowing SAS University Edition to run on PCs, Apple, and Linux computers—that is, you need to download and install something called virtualization software. If you are not familiar with this term, you are not alone. This author was a complete novice using virtualization software when my book, An Introduction to SAS University Edition, was written. Since then, especially with the help of my younger son, I am much more comfortable with these tools.
A virtual computer is a computer that runs on your real computer, running its own operating system and accessing files on your real computer (this is the tricky part). In the PC (Windows) environment, you have several choices of virtualization software. SAS is now recommending a product called Oracle VM VirtualBox. You can download this for free for your own use. This is an “open source” product that is supported by Oracle Systems, and it supports all of the operating systems mentioned.