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Description of work
Our objective within this longitudinal study has been to advance our understanding of the nature of spoken language impairment in general, with a particular emphasis on Specific Language Impairment (SLI). This research has been concerned with three related issues.

Individual Differences in Language Acquisition
The first domain begins with the obvious: Children differ with respect to their capacity to acquire language. Thus, there are individual differences among children in the rate, and possibly the kind of language acquisition. This research program examines these individual differences in language acquisition across development. Therefore, we will be able to measure individual differences in growth rates and patterns of growth across several domains of spoken and written language.

Individual differences are necessary conditions for any notion of abnormal function; however, we contend that individual differences by themselves do not constitute impairment. Furthermore, the nature of the individual differences - whether they are qualitative or quantitative differences are often assumed to provide evidence of disease. However, eye color and blood types are qualitative forms of individual differences that are not construed to be disease. In order for some form of individual differences to be viewed as an impairment it is necessary for these differences to impact negatively on the person's life.

Social/Functional Costs
If we are to claim that children with certain language skills present language impairment it is necessary for us to show that there are negative outcomes associated with these language skills. Thus, a second domain of the proposed research recognizes that language enables individuals in our society to meet many of the demands society places upon them. Failure to meet these demands, due to limitations in language proficiency leads to undesirable outcomes in life function. One of the most prominent impairments is reading. We are continuing to examine this relationship between spoken and written language. In so doing, we hope to provide better explanations regarding the contributions of spoken language to reading disorders, and furthermore, to provide a means to evaluate diagnostic standards for spoken language disorder.

Causation
The third domain of our research recognizes a need to examine prominent theories concerned with the causes of those individual differences that comprise language impairment. We recognize that the etiology of language impairment is likely complex and varied. Thus, the adequacy of explanatory accounts of SLI may be influenced by the manner in which the diagnosis is made and thus there may be subtypes of SLI that can be revealed when important causal factors are examined in association with different clinical manifestations including differential patterns of growth. Within the research proposed are several studies designed to examine prominent theoretical explanations of SLI that derive from cognitive psychology. These theoretical accounts of SLI predict that the language deficits of these children can be found in limitations in general purpose cognitive systems that are particularly crucial, but not unique to language development. These systems concern limited capacities in memory and attention and/or speed of information processing. An alternate account of SLI has been that these language problems are rooted in language specific cognitive systems.

Our research program is also providing important insights into the role of language as a causal agent for reading impairment and also the effect of reading on later spoken language. Thus, within the limits of an observational design, we will be examining the relationship between spoken language and reading as potential reciprocal determinants. The direction of this influence is likely to shift across development such that spoken language at school entry may affect later reading development.

Longitudinal Study Description

Cohort Characteristics. The longitudinal cohort that has provided the participants for this research was obtained from a large sample of kindergarten-age children who participated in a cross-sectional epidemiologic study of SLI. A description of the sampling methods for the original cross-sectional sample can be found in Tomblin, et al. (1997) and the methods for selection of the longitudinal sample is provided in Tomblin, Zhang, & Buckwalter (2000). The members of this cohort, who initially consisted of 604 children, had been diagnosed with respect to language impairment when they were in kindergarten. These children were then re-evaluated two, four, eight and now ten years after kindergarten. Figure 1 provides sample size, age and the years in which these children participated at each observation. This figure shows that the observation interval covered a two-year time span. This is due to the fact that the participants were initially sampled over a two-year time period thus at any time, the total cohort covers a two-year age range. This allows us to distribute the burden of data collection for any interval over a two-year time period.

Figure 1. Ages and calendar years when cohort members were observed. Values at each observation year represent number of children in the two cohorts.

The last complete data set was obtained at Year Eight and thus we do not have summary data for Year Ten at this time.

Measures Obtained During First 10 Years. During the course of this longitudinal study the members of this cohort have been systematically observed in order to document their listening, speaking, reading, and writing development. Additionally, we have obtained information from parents and teachers concerning their academic, behavioral, mental and physical health. During the current year we are also obtaining self-report information. The data we have gathered characterize the development of our cohort children between kindergarten and 10th grade. This database is substantial, containing over 2,215 variables. In addition to data that pertains to the children in the cohort, the parents have provided data concerning themselves, their rearing practices, and certain home characteristics. The teachers have also provided data concerning the nature of the classroom and curriculum.