This article is the second in a two-part series designed to help you better understand cancer research. It outlines various types of study designs and provides tips for evaluating study results. Part I  describes the publishing process, the format that journals and other scientific publications use to share findings, and how to find studies of interest to you.
Researchers can design medical studies in different ways, depending on the question they want to answer and the best way to answer it. No study design is perfect; each has strengths and drawbacks. When evaluating the results of a study, it is important to know its design so that you know if the results apply to your situation.
In cancer research, there are two main categories of research study design:
- Experimental studies
- Observational studies
In an experimental study, researchers apply an intervention (such as a treatment) to a group of individuals, and compare the result to that of another group that does not receive the intervention. The researchers have control over who receives the intervention and who does not, making the assignments either randomly (for reasons explained below) or through intentional selection. Meanwhile, in an observational study the researchers observe groups in which the intervention that each person receives is not controlled by the researchers.
Experimental studies can include treatments, correlative science (testing whether specific genes or proteins affect the development or spread of cancer), and quality of life issues. Meanwhile, observational studies tend to be epidemiologic (relating to how various risk factors cause or affect disease in a population).
Types of experimental studies
Experimental studies are generally considered more reliable than observational studies because, in most cases, the people in an experimental study are randomly assigned to the intervention or control groups, reducing the chance of having the researchers' or the subjects' biases (assumptions or preferences) affect the study results. In addition, with an experimental study, the researchers can better account for and control factors âsuch as age, sex, and weightâthat could affect the results of the study.
One of the most common types of experimental studies is the clinical trial . A clinical trial is a medical research study performed with people that tests the safety or effectiveness of a new drug, combination of existing treatments, approach to radiation therapy or surgery, or method of cancer prevention.
Doctors and researchers conduct clinical research in segments called phases. Each phase of a clinical trial is designed to provide different information about the new treatment, such as the dose, safety, and efficacy (how well it works). The phases are described as I, II, and III. Learn more about the phases of clinical trials .
Depending on experimental factors, researchers can “blind” (or shield) clinical trial study participants and/or themselves from knowing whether specific study participants belong in the intervention or control group, eliminating subjective bias. Types of experimental studies include:
Double-blind randomized trial. This type of clinical trial is considered to produce the best evidence because neither the study subjects nor the researchers know who belongs to the intervention or control group until the study ends.
Single-blind randomized trial. In this type of trial, individual subjects do not know whether they belong to an intervention or control group, but researchers do.
Non-blind trial. In this type of trial, both subjects and researchers know who belongs to each test group within the study.
Types of observational studies
In observational studies, researchers have less control over the characteristics of the study participants, which means that certain factors could unknowingly affect the results. These studies, however, are useful in providing preliminary evidence that can help guide future experiments.
Types of observational studies include:
Case-control studies. These observational studies compare two groups of people, such as those who have cancer (the case) and those who do not (the control). For example, researchers may look for lifestyle or genetic differences between the two groups that may explain why one group has cancer and the other does not. These studies are done retrospectively, meaning that the circumstances that they are evaluating have already happened.
Cohort studies. These studies are prospective, which means that the event is studied as it occurs. The researchers monitor a group of people for a long time and track, for example, any new cases of cancer. This approach is often used to study whether certain nutrients or behaviors can prevent cancer. Also, this approach can be used to identify cancer risk factors, such as the link between the use of postmenopausal hormone replacement therapy and an increased risk of breast cancer.
Case-series studies. These studies are compilations of detailed descriptions of a patient's diagnosis and treatment history. These individual patient descriptions are called case reports. If many patients are given a similar treatment, each case report may be combined to form a case series. Outcomes from case-series studies are descriptions of patients' experiences within a specific population and should not be used to determine treatment options.
Cross-sectional studies. These studies examine the relationship between diseases and other factors (such as exposures or behaviors) within a specific population at one point in time. However, because these studies only measure circumstances at one point in time, they cannot make strong cause-effect claims.
Meta-analyses. These studies combine the results of several studies on the same topic. By combining studies, a meta-analysis has the ability to find trends that may not be apparent in smaller studies. However, if the individual studies are poorly done, the results of a meta-analysis may not be useful.
Evaluating research studies
In addition to considering the study design, here are some tips for evaluating the information you find in a research study:
Find out if the journal uses a peer-review process. Results from a study are more reliable if they are peer-reviewed, meaning that other researchers not affiliated with the study have approved the design and methods.
Look at the length of the study and the number of people involved. A study is more applicable and believable if the same results occur in many people across a long time. An exception to this rule can be made for studies of rare cancer types or cancers with a poor prognosis or chance of recovery because there may only be a small number of patients to study. Also, when considering the length of the study, it may be appropriate for some clinical trials to be shorter. For instance, cancer prevention trials are often much longer than treatment clinical trials.
Determine if the study supports or contradicts information that is already available. New results are exciting, but other researchers must validate the results before the medical community accepts them as fact. Review articles known as “systematic reviews” may be of particular interest because they provide an analysis and draw conclusions across all the published research.
Watch out for conclusions that overstate the results. Each study is a small piece of the research puzzle, and medical practice rarely changes because of the results of one study.
Talking with your doctor
Even if you have reviewed a reputable study that suggests a different approach to cancer treatment, do not stop or change your treatment before talking with your doctor about the information you find in an abstract or study and discussing treatment options together.
Some suggested questions to ask your doctor include:
- I recently heard about a study that tried a new treatment. Is this treatment applicable to my type and stage of cancer?
- What type of journals should I read to learn more about my type of cancer?
- Should I consider being a part of a clinical trial?
- What clinical trials are open to me?
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Last Updated: November 01, 2010