Section 1: Chapter 3:

USMLE Biostatistics: Precision, Accuracy, Reliability, Internal Validity & External Validity

Accuracy, Precision (reliability), & Validity all describe vital characteristics of a study. It is essential to have a thorough understanding of these concepts for both the USMLE/board exams and for applying research to clinical practice.

Chapter Objectives

In the simplest of terms, precision, accuracy and validity describe how “good” and how “reproducible” the results or findings of the study were. Physicians and scientist must consider all of these metrics when utilizing medical literature and especially when applying research findings to clinical practice. For this reason, these are essential topics which are tested in medical schools and board exams everywhere. In this short video lecture, we will perform an in depth analysis of these topics and pay particular attention to the following concepts.

  • Definition of accuracy; what is accuracy in terms of research?
  • Definition of precision; what is precision in terms of research?
  • Definition of validity; what is validity in terms of research?
  • Types of validity; what is internal validity and external validity?
  • Threats to internal validity and external validity (aka systematic error or bias and random error or chance)
  • Assessing the level of validity and precision in studies; How do we measure validity and precision in studies?
  • Methods to increase validity and precision; How do we ensure that studies have high levels of validity and precision?
By the end of this chapter, you should have a basic understanding of the essential metrics used to describe the quality of research findings. In this next chapter, chapter 4, we will discuss another essential component of health research which you must be familiar with for the USMLE Step 1 biostatistics portion. This essential component is samples and populations in research. As we will soon learn, the sample population used in research studies have a huge impact on the quality and applicability of the study.

Chapter Details 

  • Video length: 22 min 
  • Practice exercises: 8 USMLE style multiple choice questions with in-depth explanation
  • Estimated time to complete video lecture and practice exercises: 40 min 
  • Next chapter: Chapter 4 – Samples, Populations and Randomization



Fletcher, G. S. (2012). Clinical epidemiology: The essentials [5th Edition]. Philadelphia ; Baltimore ; New York: Wolters Kluwer. Retrieved from

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