“Which Came 1st–The chicken or the egg?” (or, Why Correlation is Not Causation)

Correlation is not causation. RNs who want to use research in practice must take this seriously.

What does it mean?   Answer: Just because two things happen together, we cannot say that one causes the other.

Consider the example of drinking coffee and staying awake. The more coffee you drink, the more hours you will stay awake.   But isn’t it also true that the more hours you try to stay awake, the more coffee you will likely drink?

Thus, in a study about coffee drinking and sleep, you may read that coffee and hours of being awake are correlated. In other words, they occur together. When one goes up, the other goes up.   What is not clear is whether the coffee is causing the person to be awake longer, OR whether being awake longer is causing the effect of more coffee consumption.   The unsolved mystery is: “Which is the cause and which is the effect?”[1]

Likewise consider the consistent relationship between chickens and eggs. Every egg was produced by a hen. Every one. In statistical terms this means that on a scale of 0 to 1 (with 0 being no relationship whatsoever and 1 being a relationship that occurs 100% of the time) eggs and chickens have a perfect 100% relationship of 1. (A statistician would write this as r=1.0).   What is unclear is whether (when the world was young), the chicken appeared first and caused the first egg, or the egg came first and caused the first chicken. Again the unsolved mystery is: “Which is a cause and which is the effect?”

Okay, so let’s do some critical thinking about actual research.  You read these results:

“More calls for assistance related to less fall-related patient harm. Surprisingly, longer response time to call lights also related to fewer total falls and less fall-related patient harm. Generally speaking, more call light use related to longer response times.”[2]

When you read this article, what should you be assuming about the researchers’ findings in terms of relationships instead of cause-and-effect? (Hint: Think about chickens & eggs, or coffee & insomnia.)

[1] Bonus info: We call causes “independent variables” and we call effects, “dependent variables”

[2] Tzeng, H.M,. & Yin, C.Y. (2009). Relationship between call light use and response time and inpatient falls in acute care settings. Journal of Clinical Nursing, 18(23), 3333-3341. doi: 10.1111/j.1365-2702.2009.02916.x. Epub 2009 Sep 4.

Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s