What is a logit?

What is a logit?  With the new passing standard for RNs and the potential for an upcoming change in the NCLEX-PN® passing standard, there is a flurry of questions regarding this strange but important word.  I would like to offer a short “Plain English” discussion on the logit.

When I try to think about or explain a logit I use the analogy of the PT to the INR.  The Prothrombin Time (PT) is the time it takes for blood to clot, and is much like the score a candidate receives on the NCLEX.  To calculate the PT, laboratories use a control value, and this value changes with each batch of solutions delivered, much like the average level of difficulty on each candidate’s NCLEX.  The International Normalizing Ratio (INR) uses a formula that takes into account these two variables, much like the logit takes into account its two variables.  The logit value, like the INR value, takes into account the variables for every candidate taking the NCLEX and then “evens them out.”  This provides consistent comparison of candidates much like the INR provides consistent data on clotting.

So think about it in this context as we move along.

A logit is a name given to the product of a formula.  The name comes from the math world, and represents a logistic function.  Since nurses have a language all our own I guess mathematicians can have one also!  The formula that provides the logit considers the percent of questions correct by a candidate and the average level of difficulty of those questions.  This is the formula:

Candidate ability (the logit score)= [% correct/1-% correct ] +  Average Difficulty*

Using the formula (you remember stats class right?) “normalizes” the data.  The formula allows students to answer different questions that are the same level of difficulty and have them evaluated with the same parameters.  If the formula did not work, the NCSBN could not use different questions; every candidate would have to take the same test, and that test would not be secure for very long!

Logit

I visualize the scale as a standing yard stick that goes to infinity, but for ease of consideration let’s look -2 to +2.   When the passing standard is evaluated every three years, the panel of judges and then the NCSBN board looks at the previous passing standard, the pass rate, and feedback from a survey of employers and educators.   The questions are: “Is the passing standard protecting the population?” and “Are we giving licenses to safe nurses?”   The passing logit represents the voted on level of difficulty of questions and percent correct at this given point in time.   Because students change, curriculum changes, and the type of practice “new nurses” perform changes, this makes it difficult, if not impossible, to predict with certainty what a passing standard change will DO.

In 1994 the first passing standard was -0.4766 logits.  Now in 2013 the passing logit is 0.   This is interesting to look at, and the view totally depends on how big you make the picture.  When you look at it the numbers alone, it does not seem like much of a jump, but if you only look at a scale of -1 to +1, the jump appears bigger.  The first year of candidates taking the recalibrated NCLEX will tell us how this change will affect the overall passing standard.

“It is what it is,” I tell my students.  They will take the NCLEX-RN®, and many will succeed.  Those who are best able to pass will be the students ready to start the test at a higher minimum competency level and best prepared to answer questions at a higher level of difficulty.

 

*Source: National Council of State Boards of Nursing. https://www.ncsbn.org/What_is_a_Logit.pdf

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