Basics of study design: Practical considerations

From the “Biostatistics and Epidemiology Lecture Series, Part 1”

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Basic research skills are not acquired from medical school but from a mentor.1,2 A mentor with experience in study design and technical writing can make a real difference in your career. Most good mentors have more ideas for studies than they have time for research, so they are willing to share and guide your course. Your daily clinical experience provides a wealth of ideas in the form of “why do we do it this way” or “what is the evidence for” or “how can we improve outcomes or cut cost?” Of course, just about every study you read in a medical journal has suggestions for further research in the discussion section. Finally, keep in mind that the creation of study ideas and in particular, hypotheses, is a mysterious process, as this quote indicates: “It is not possible, deliberately, to create ideas or to control their creation. What we can do deliberately is to prepare our minds.” 3 Remember that chance favors the prepared mind.


Often, the most difficult task for someone new to research is developing a practical study idea. This section will explain a detailed process for creating a formal research protocol. We will focus on two common sticking points: (1) finding a good idea, and (2) developing a good idea into a problem statement.

Novice researchers with little experience, no mentors, and short time frames are encouraged not to take on a clinical human study as the principle investigator. Instead, device evaluations are a low-cost, time-efficient alternative. Human studies in the form of a survey are also possible and are often exempt from full Institutional Review Board (IRB) review. Many human-like conditions can be simulated, as was done, for example, in the study of patient-ventilator synchrony.4,5 And if you have the aptitude, whole studies can be based on mathematical models and predictions, particularly with the vast array of computer tools now available.6,7 And don’t forget studies based on surveys.8

A structured approach

A structured approach for developing a formal research protocol.

Figure 1. A structured approach for developing a formal research protocol.

A formal research protocol is required for any human research. However, it is also recommended for all but the simplest investigations. Most of the new researchers I have mentored take a rather lax approach to developing the protocol, and most IRBs are more interested in protecting human rights than validating the study design. As a result, much time is wasted and sometimes an entire study has to be abandoned due to poor planning. Figure 1 illustrates a structured approach that helps to ensure success. It shows a 3-step, iterative process.

The first step is a process of expanding the scope of the project, primarily through literature review. Along the way you learn (or invent) appropriate terminology and become familiar with the current state of the research art on a broad topic. For example, let’s suppose you were interested in the factors that affect the duration of mechanical ventilation. The literature review might include topics such as weaning and patient-ventilator synchrony as well as ventilator-associated pneumonia. During this process, you might discover that the topic of synchrony is currently generating a lot of interest in the literature and generating a lot of questions or confusion. You then focus on expanding your knowledge in this area.

In the second step, you might develop a theoretical framework for understanding patient-ventilator synchrony that could include a mathematical model and, perhaps, an idea to include simulation to study the problem.

In the third step, you need to narrow the scope of the study to a manageable level that includes identifying measurable outcome variables, creating testable hypotheses, considering experimental designs, and evaluating the overall feasibility of the study. At this point, you may discover that you cannot measure the specific outcome variables indicated by your theoretical framework. In that case, you need to create a new framework for supporting your research. Alternatively, you may find that it is not possible to conduct the study you envision given your resources. In that case, it is back to step 1.

Eventually, this process will result in a well-planned research protocol that is ready for review. Keep in mind that many times a protocol needs to be refined after some initial experiments are conducted. For human studies, any changes to the protocol must be approved by the IRB.

The problem statement rubric

The most common problem I have seen novices struggle with is creating a meaningful problem statement and hypothesis. This is crucial because the problem statement sets the stage for the methods, the methods yield the results, and the results are analyzed in light of the original problem statement and hypotheses. To get past any writer’s block, I recommend that you start by just describing what you see happening and why you think it is important. For example, you might say, “Patients with acute lung injury often seem to be fighting the ventilator.” This is important because patient-ventilator asynchrony may lead to increased sedation levels and prolonged intensive care unit stays. Now you can more easily envision a specific purpose and testable hypothesis. For example, you could state that the purpose of this study is to determine the baseline rates of different kinds of patient-ventilator synchrony problems. The hypothesis is that the rate of dyssynchrony is correlated with duration of mechanical ventilation.

Here is an actual example of how a problem statement evolved from a vague notion to a testable hypothesis.

Original: The purpose of this study is to determine whether measures of ineffective cough in patients with stroke recently liberated from mechanical ventilation correlate with risk of extubation failure and reintubation.

Final: The purpose of this study is to test the hypothesis that use of CoughAssist device in the immediate post-extubation period by stroke patients reduces the rate of extubation failure and pneumonia.

The original statement is a run-on sentence that is vague and hard to follow. Once the actual treatment and outcome measures are in focus, then a clear hypothesis statement can be made. Notice that the hypothesis should be clear enough that the reader can anticipate the actual experimental measures and procedures to be described in the methods section of the protocol.

Here is another example:

Original: The purpose of this study is to evaluate a device that allows continuous electronic cuff pressure control.

Final: The purpose of this study is to test the hypothesis that the Pressure Eyes electronic cuff monitor will maintain constant endotracheal tube cuff pressures better than manual cuff inflation during mechanical ventilation.

The problem with the original statement is that “to evaluate” is vague. The final statement makes the outcome variable explicit and suggests what the experimental procedure will be.

This is a final example:

Original: Following cardiac/respiratory arrest, many patients are profoundly acidotic. Ventilator settings based on initial arterial blood gases may result in inappropriate hyperventilation when follow-up is delayed. The purpose of this study is to establish the frequency of this occurrence at a large academic institution and the feasibility of a quality improvement project.

Final: The primary purpose of this study is to evaluate the frequency of hyperventilation occurring post-arrest during the first 24 hours. A secondary purpose is to determine if this hyperventilation is associated with an initial diagnosis of acidosis.

Note that the original statement follows the rubric of telling us what is observed and why it is important. However, the actual problem statement derived from the observation is vague: what is “this occurrence” and is the study really to establish any kind of feasibility? The purpose is simply to evaluate the frequency of hyperventilation and determine if the condition is associated with acidosis.


Next Article:

Chi-square and Fisher’s exact tests

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