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Interpreting NWEA Growth Norms

By MikeD on June 23, 2010 at 10:00 am

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Last year NWEA published an updated version of the RIT scale norms document.  The new report improves upon the prior one with more schools, more students, and improved statistical techniques for predicting normative growth.  But what do these normative growth estimates really mean?

Growth Estimates: Target or Typical

Occasionally one may hear the NWEA growth estimates referred to as growth “targets”, a somewhat misleading term, since it implies that children who meet them are “on track”, or that the growth estimates are connected somehow to specific educational outcomes.  They are not.  Students who meet NWEA growth estimates aren’t necessarily on track for meeting state proficiency standards, nor are they necessarily on track for college readiness.  NWEA growth estimates are not connected to any external measures of educational success.  They simply tell us how much growth was typical (approximately average) for a group of similar students within the norming sample.  Technically speaking, the growth estimates aren’t really averages because the latest norm study uses more sophisticated statistical models than that.  But that’s essentially what they represent:  average growth for a particular group of kids within the norm sample.

So, again, what do NWEA growth estimates really mean?

They are our best guess for how much growth the typical kid will show over the specified time interval, given her/his enrolled grade and initial RIT score.  Typically, about half of the kids in any school will show growth that meets or exceeds their specific growth estimates, and about half will not.  Why?  For the same reason that about half of the people in a typical room are taller than the U.S. average height and about half are shorter.  That’s how averages work, at least under usual circumstances.

Unreached Growth Targets

Nobody should be too surprised or concerned if an individual person in our hypothetical room was below average height.  But if nearly everyone, or if significantly more than half of the people were below average height, this might suggest something unusual about the room.  NWEA growth estimates work the same way.  Nobody should wonder when an individual fails to meet her/his growth estimate, since the law of averages suggests that about half of students will not.  But if far more than 50% of the students within a group fail to meet their growth estimates, then we may suspect that something strange is going on.  Similarly, it would be just as unusual for significantly more than half of to meet their respective growth estimates (unless the school is situated near Lake Woebegone, MN).  Both situations can provide us with information about how well a school is doing.

The growth estimates within our latest norming study provide great information about how to evaluate groups of students, provided those students are comparable to the students within the norming study.  When we wish to evaluate students whom we believe are quite different from our norming group, then we need to employ other methods.  But that’s another story…


Michael Dahlin is one of the key research specialists at NWEA. His focus is on research related to educational assessment, standardized testing, and predictors of educational achievement.

2 comment(s) - you must be logged in to comment

Mike Dahlin's picture
Remember, it's important to distinguish between the performance of an individual and the average performance of the group. Are you saying that the average growth of your group of fourth graders is lower than the average fourth grade growth target? If so, by how much? My reading of the norms document indicates that among fourth graders, the average growth in mathematics from beginning of school year to end of school year is about 8.86 RIT points (with a standard deviation of about 10.72). The relatively large standard deviation means that there is a lot of variability around that growth target. A simple effect size calculation may help to provide some context over whether your group's average is meaningfully lower. Typically, effect sizes lower than about 0.2 are considered very small. So if your fourth graders' average growth was between 6.72 and 8.86, then you probably need not be all that worried.

Kathy Thorsen's picture
Are you saying that we should not be concerned if all grades hit growth target except for grade 4 in Math? Does this not imply the need for a conversation about fourth grade math?

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