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Well, I don't know but I'll chime in with some citations anyway. My guess is that the rate of incidence is dependent on a number of other variables. Thus, you would expect to see a higher incidence in some populations than others.
The evidence is mixed on this topic. Frank Vellutino is one of the major contributors to this line of research. Studies in several US states done by Vellutino (Albany, NY), Torgesen (Fla), someone in Texas -- I forget who-- and elsewhere came up with fairly consistent findings that explicit instruction in reading, including code knowledge, blending and advanced phonics skills reduced the number of students with decoding disabilities to under 10%. (The range was 3%-10%).
Vellutino's studies, and some others, did not include children with IQ's below 90. Decoding difficulties, per se, are not correlated with IQ.
Different programs were used, as well as ad-hoc instruction that drew on various sources and materials but was focused and explicit. No particular program or approach clearly surpassed others -- some were more suited to 1:1 instruction, others to group instruction.
Gough and others published a large-scale study that showed more than 95% of children in first and second grades could be taught effectively enough to score above the 25th percentile, which is considered in the average range.
One problem with all these studies, however, is that they lack longitudinal data. Children who successfully master basic decoding skills and are reading well in first and second grade may fall behind later for a variety of reasons (no single cause). Some continue to struggle with decoding skills at higher levels, others with syntax, language comprehension (including, but not limited to, vocabulary), fluency, auditory and working memory, and more. While poor readers in second grade rarely end up as outstanding readers in fifth grade, the opposite can happen: the good reader in second grade can be a failing reader in fifth and beyond.
Some of Vellutino's articles:
Vellutino, F. R., Scanlon, D. M., Sipay, E.R., et al. (1996). "Cognitive profiles of difficult-to-remediate and readily remediated poor readers: Early intervention as a vehicle for distinguishing between cognitive and experimental deficits as basic causes of specific reading disability." Journal of Educational Psychology 88(4): 601-638.
Vellutino, F. R., Scanlon, D. M., Snowling, M. J., & Fletcher, J. M. (2004). "Specific reading disability (dyslexia): What have we learned in the past four decades?" Journal of Child Psychology Psychiatry 45(1): 2-40.
Scanlon, D. M., & Vellutino, F. R. (1997). A comparison of the instructional backgrounds and cognitive profiles of poor, average, and good readers who were initially identified as at risk for reading failure. Scientific Studies of Reading, 1, 191- 215.
Vellutino, F. R., Scanlon, D. M., and Tanzman, M. S. (1994). Components of reading ability: Issues and problems in operationalizing word identification, phonological coding, and orthographic coding. In G. R. Lyon (Ed.), Frames of reference for the assessment of learning disabilities: New views on measurement issues (pp. 279-329).
Other sources make the point that low-achievers (who may find reading difficult for varying reasons) need much more instructional time -- at least twice as much, sometimes 3 times as much, and in a more intensive, fast-paced, interactive milieu. This is rarely provided and probably not even possible in most public school settings. The only longitudinal accounts of such success are from the UK, and the details of what was done and when and how in both the Clackmannanshire and W. Dunbartonshire studies are sketchy.
Nothing similar has taken place on this side of the Atlantic; the best overall results have been from DI schools, but even there, population mobility, staff turnover and limitations on instructional time lower the ceiling for student achievement.
Accelerating Reading Attainment: The Effectiveness of Synthetic Phonics by Joyce E. Watson and Rhona S. Johnston (pdf file)
One Man's Quest to Eliminate Illiteracy
West Dunbartonshire's Literacy Scheme
4 comments:
Gough and others published a large-scale study that showed more than 95% of children in first and second grades could be taught effectively enough to score above the 25th percentile, which is considered in the average range.
Ah, statistics. Aren't they fun?
How on earth can 5% of students make up the whole bottom 25%ile of readers?
It's not 5% of the whole school population, just 5% of this exceptionally well-taught sample.
I understood the statistic to mean that 5% of the students in the study scored below the 25th %ile on national norms.
I found some more interesting statistics and estimates in a statement by Reid Lyon in 2001 to a Congressional subcomittee on education reform. I'll post it in 2 parts:
"I am pleased to have been asked to address the Subcommittee on issues relevant to the use of assessments and accountability to raise student achievement, particularly with respect to how these issues and our NICHD reading research findings are reflected in President Bush's reading initiatives. It is also timely that you have requested information about how scientifically based early reading instruction will reduce the need for special education. Recently, Dr. Jack Fletcher of the University of Texas Health Science Center in Houston and I completed such an analysis. I am happy to share those findings with you today.
As you know, the NICHD considers that teaching and learning in today's schools reflect not only significant educational concerns but public health concerns as well. Our research has consistently shown that if children do not learn to understand and use language, to read and write, to calculate and reason mathematically, to solve problems, and to communicate their ideas and perspectives, their opportunities for a fulfilling and rewarding life are seriously compromised. Specifically, in our NICHD-supported longitudinal studies, we have learned that school failure has devastating consequences with respect to self-esteem, social development, and opportunities for advanced education and meaningful employment. Nowhere are these consequences more apparent than when children fail to learn to read. Why? Simply stated, the development of reading skills serves as THE major foundational academic ability for all school-based learning. Without the ability to read, the opportunities for academic and occupational success are limited indeed. Moreover, because of its importance, difficulty in learning to read crushes the excitement and love for learning, which most children have when they enter school.
As we follow thousands of children with reading difficulties throughout their school careers and into young adulthood, these young people tell us how embarrassing and devastating it was to read with difficulty in front of peers and teachers, and to demonstrate this weakness on a daily basis. It is clear from our NICHD research that this type of failure affects children negatively earlier than we thought. By the end of first grade, children having difficulty learning to read begin to feel less positive about themselves than when they started school.
(continued)
As we follow children through elementary and middle school years, self-esteem and the motivation to learn to read decline even further. In the majority of cases, the students are deprived of the ability to learn about literature, science, mathematics, history, and social studies because they cannot read grade-level textbooks. Consider that by middle school, children who read well read at least 10,000,000 words during the school year. On the other hand, children with reading difficulties read less than 100,000 words during the same period. Poor readers lag far behind in vocabulary development and in the acquisition of strategies for understanding what they read, and they frequently avoid reading and other assignments that require reading. By high school, the potential of these students to enter college has decreased substantially. Students who have stayed in school long enough to reach high school tell us they hate to read because it is so difficult and it makes them feel "dumb." As a high school junior in one of our studies remarked, "I would rather have a root canal than read."
It is important to note that this state of educational affairs describes an extraordinary and unacceptable number of children. According to the National Center for Educational Statistics (1998), 38 percent of fourth graders nationally cannot read at a basic level--that is, they cannot read and understand a short paragraph of the type one would find in a simple children's book. Unfortunately, reading failure is disproportionately prevalent among children living in poverty. Indeed, in many low income urban school districts the percentage of students in the fourth grade who cannot read at basic level approaches 70 percent.
But it is important to understand at the outset that the number of children with reading difficulties served in special education reflects only a fraction of the number of school age children who fail to learn to read. Recall from the previous discussion that 38% of fourth grade students read below the basic level. Keeping in mind that the majority of these children will continue to have reading difficulties throughout their school career if they do not receive systematic and focused early intervention, we can estimate that at least 20 million school age children suffer from reading failure. Among these 20 million children, only approximately 2.3 million school-age children are served in special education under the category of learning disabilities (LD). The remaining 17.7 million poor readers not meeting the eligibility requirements for the LD category are either provided some form of compensatory education or overlooked all together.
We have taken care in our NICHD early intervention and prevention studies to identify ALL children who are at-risk for reading failure within a given sample and to identify the instructional approaches that are the most effective for the majority of these students, irrespective of whether they are eligible for special education as an LD student or eligible for compensatory education services. As noted earlier, these studies have indicated that with the proper early instruction, the national prevalence of reading failure can be reduced significantly. Thus, by putting in place well designed evidence-based early identification, prevention, and early intervention programs in our public schools, our data strongly show that the 20 million children today suffering from reading failure could be reduced by approximately two-thirds. While still a totally unacceptable rate of reading failure, such a reduction would allow us to provide services to the children who are in genuine need of special education services with substantially greater focus and intensity. "
I've had the privilege of hearing Dr. Lyon speak on two occasions and was very favorably impressed, not only by his knowledge but by his very evident passion for improving the lives of children through good reading instruction.
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