NVLD - Where does the 15 point limit come from? (Rewritten)

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NVLD is often defined as VIQ-PIQ beeing over 15 points.

Where does this limit come from? (1)

Is it a practical limit for classroom NVLD, or is it more like a tipping point for a wide range of problems?

Is it a lower limit (2a), with perhaps some additional criteria needing to be satisfied, or is it more like a tipping point, where the population size corresponds to the population with a higher difference, but some below does have it and some above does not have it?(2b)

Is PIQ (rather that NVIQ for example), used because it is most important in the classrom, or because it is more predictive of wider problems.

If 2b : I am aware that more specific problems can lead do diagnosis below the limit. Is this factored into the limit?



Confusing first version

NVLD is often defined as the PIQ beeing in the lower 2.5% percentile of the population compared to the VIQ.

Where does this limit come from?

Is it a practical limit for classroom NVLD, or is it more like a tipping point for a wide range of problems?
(Like is the difference between 2.3 and 2.7 quite big compared to the difference between 2.7 and 3.1?)

Valentine

Last edited by mrsmith on 29 Nov 2012, 9:22 am, edited 10 times in total.

I've not heard those limits. I've heard it's more described by PIQ to VIQ gap. I've heard a 15 point gap between your VIQ and PIQ is enough to satisfy diagnostic criteria. In my case, my gap is around 50 points. My PIQ is like 80, and my VIQ is like 130+. So it's rather severe NVLD, I guess. But I think officially, it starts at 15 point gap.

It comes from 2 standard deviations, which I think corresponds to a certain point difference the way the IQ scale is defined.
(I think 15points is 1 stdev - corresponding to the 15% percentile).

I think it is better to think of it in terms of percentiles - You can compare it to the 1% who has ASD or the numbers for ADHD.

but what about in the weschler children scale 4 which doesnt have performance and verbal iq its divided into 4 parts?

The definition of NVLD as a neuropsychiatric condition was first described by Dr. Bryan Rourke. Statistically speaking, though, yes, the VIQ/PIQ split comes from standard deviation for the WAIS. 15 points is 1 standard deviation, and a 15-point difference between VIQ and PIQ (where VIQ is larger) is the baseline definition for NVLD. The "four parts" Ravenclawgurl mentions are subscores, from which VIQ, PIQ, and FSIQ (VIQ + PIQ average) are determined. On the WAIS (I don't know much about the WISC), Vocabulary and Working Memory are used to determine VIQ, and Perceptual Organization and Processing Speed are used to determine PIQ.

Also, in order to qualify for an NVLD diagnosis, you needn't have a PIQ that is in the lowest 2%, which is -2 standard deviations from the mean of 100. (Thus, a PIQ of 85 or below.) You just need to have a VIQ/PIQ split that is a statistically significant difference (i.e., 15+ points). For example, I have NVLD, but my PIQ is 116, which is in the "high-average" range. My VIQ is 134 ("very superior" range), which is 17 points higher than my PIQ. Even though my PIQ is +1 SD from the mean of 100, I still have NVLD due to the incongruence between my PIQ and VIQ.

For NVLD, the subscores sometimes are an even better determinant of the learning disorder. Working Memory, used for VIQ, involves Digit Span and Arithmetic sections, both of which have numbers. Plus, Vocabulary is an entirely verbal/language-based score, and Perceptual Organization is an entirely visual-spatial score. I feel my NVLD staggered abilities are best shown by these two subscores: My Vocabulary score is 140 (almost +2 SD above mean), and my Perceptual Organization score is 105 ("average" range). That's a 35-point difference, compared to just 17 points for VIQ vs. PIQ.

well if thats thats how they determine performance iq with the processing speed then i likely have nvld because my processing speed score was so low it was in the MR range

I have Googled a bit, and I generally see the 1 stdev limit
That would correspond to 15% of the population, which much really be far to high, so it must be some additional filtering going on?

The only prevalence numbers I have seen are 2-3%, which corresponds to 2 stdevs.

No, 15 points for 1 SD doesn't mean 15% of the population. The way that a standard normal curve works is that ~68% of people lie within +/- 1 SD of the mean (in this case, 100). And 96% of people lie within +/- 2 SDs of the mean. So, only 2% of the population have an IQ of 130 or above (+2 SDs), and only 2% of the population have an IQ below 85 (-2 SDs).

It also isn't true that having greater than or equal to 1 SD between your VIQ and PIQ scores is equivalent to having a PIQ score that is greater than or equal to 1 SD below the mean. As I showed above, my PIQ is 1 SD above the mean, but I still have NVLD, due to its relationship to my VIQ score. Now, I'm sure that most NVLD-ers out there do have a PIQ in the 80-90 range; I realize that I am not a typical case. (I am fortunate enough to have taught myself certain visual-spatial skills over the years.) But again, it is possible to have NVLD with any level PIQ score, so long as it is 15+ points lower than the VIQ score.

As far as prevalence rates of NVLD, what I commonly read is that NVLD makes up around 1-10% of all total learning disorders. It's not all that common, even in the learning disorder category, but again, that statistic doesn't refer to specific IQ scores.

And in response to Ravenclawgurl, Symbol Coding/Processing Speed is only one component of PIQ. Block Design, Matrices, and other tests also are factored in for PIQ. Having low Processing Speed doesn't necessarily mean NVLD. I scored extremely highly in Processing Speed, yet I have NVLD.

yeah but what i mean im sure the processing speed numbers bring the rest of the performance iq down

I didn't say that. If 68.2% is within, 31.8 % is outside, or 15.9% is on each side.

Okay, I see what you meant now. But it still doesn't mean that people who qualify as having NVLD will necessarily have a PIQ score that is in that 15% below 100.

If scores are above 100% you can't say that you have a LD or that you are impaired from the scores alone.
(But when I wrote OP, I though the limit was sharper, and that it was more significant)

What if someone has a 140 PIQ and 155 VIQ? This person still has NVLD, a learning disorder?

By definition, yes. No matter how high or how low a person's IQ, there should not be more than a 10-point difference between VIQ and PIQ. Obviously, the higher the IQ scores, the less a person will struggle, but they still will experience learning difficulties compared to their potential. The fictional case you described, for instance, probably wouldn't experience many struggles, but their PIQ would still be limited in reference to what their PIQ abilties SHOULD be.

Somebody with these high of scores probably wouldn't ever need to go get tested for a learning disorder due to compensatory strategies, but they still would have staggered abilities. Again, using myself as an example, I never was tested for a learning disorder as a child. I only took an IQ test and was diagnosed with NVLD in college. Because my PIQ abilties are higher, I was able to "get by" in math classes using compensatory strategies, such as my "photographic" memory. But even though I was able to be in honors math courses and get "A"s for the most part, math always has been a huge struggle for me. And teachers never understood why somebody as seemingly "smart" as I struggled so much with math. So, it's sort of a double-edged sword. If you have a higher PIQ to begin with, yes, you are fortunate enough to (most likely) achieve greater academic success for longer, but the downside is that your very real struggles are much more puzzling to teachers and people in general. It's a lot like how there are obvious advantages to being high-functioning on the spectrum, but there also are disadvantages (it's a "hidden" disability).

If the point criteria is met by 15% of the population, it should mean that only 1/3 of those that "have NVLD".
Depending on the diagnostic criteria you can imagine it makes some difference in the chance that you pass.

One other post on this forum says it should then rather be called a "learning style" than "learning disability",
but AFIAK the term NVLD is used even if all scores are high.

Another thing is that the point difference perhaps should be different at a high IQ.

Once again, just because 15% of the population has a full-scale IQ that is -1 SD from the mean does not mean that 15% of the population has a 15-point difference between VIQ and PIQ to make up that FSIQ. It is not a common thing to see greater than 10 points between VIQ and PIQ, no matter what the FSIQ is.