Correlation coefficient

According to most math books, the correlation coefficient is the linear association between two variables.  Bottom line: correlation coefficient shows the relationship (or, association) between two things.

The correlation coefficient NEVER shows or proves causation.  A USMLE high yield question may include answers with the word “cause” in it.  Never select an answer-choice with the word “cause” in it.

Pictures are worth a thousand words so the USMLEbiostatistics workbook provides pictures and arrows that highlight everything you need to know about this topic

Core concepts of the correlation coefficient

The Correlation Coefficient is a number between, -1 and 1 (including -1 & 1), and is denoted by the letter, “r” (e.g., r = 0.78).  r provides the direction & strength of a correlation between two things – the, "linear association between two variables."

Negative Correlation (lifespan ↓ as smoking ↑)

If r is between -1 & -0.1, then the direction is .
-1 = perfectly negative correlation (maximum strength), so a -0.6 represents a mildly-strong association.










Here's a general scale for Correlation Coefficient's strength:

Positve Correlation (height ↑ as weight ↑)

If r is between +0.1 & +1, then the direction is .
+1 = perfectly positive correlation (maximum strength), so a +0.8 represents a strong association.

Solving / answering high yield correlation coefficient questions

Questions may provide scatter plots (the easiest) or, entirely in the form of a word problem (trickiest).  Calculations are not required; instead, it’s entirely conceptual.

A high yield question designed to confuse the examinee could provide a positive r, p=0.02, but frame the answer so the correct choice involves two decreasing variables.  I provide practice questions that address this. 

The USMLE biostat workbook describes this topic by using many pictures, additional details, a wide variety of word-problems with thorough explanations, and much more. Buy now!

Back to Biostatistics








































Double hump

Renamed to "double chump," I guarantee that this seemingly troublesome test question will become completely intuitive and absolutely NO trouble.

Read more about the tools...

All these IMGs and medical students are saying, “Biostatistics is EASY!!!”


Simply yet thorough – read about the med students & IMGs’ experiences with the USMLEbiostat workbook of tools!



Links just for med students & IMGs.