Below, we synthesize the findings from the studies reviewed, which examine the effects of different early care and education interventions on teacher and child outcomes. This paper focuses solely on child outcomes. We begin by discussing the evidence that federally-funded research on early childhood language and literacy-specific curricula has provided in terms of identifying effective interventions for improving young children's oral language, phonological sensitivity, and print knowledge skills the three foundational skills upon which later literacy is based. We then discuss what this body of research has added to our understanding of some of the key factors that moderate the effectiveness of intervention programs. It should be noted that a challenge in reviewing this body of research was that most interventions were broad-based, encompassing many different components. This meant that, in this set of studies, when positive effects on child outcomes were found, it was often not possible to determine which of the many components was contributing to these effects. Fortunately, a more extensive review to be released soon the National Evaluation of Early Literacy (NELP) will be able to provide some insight into this question.
As more and more young children spend large portions of their time with teachers in early education settings, the quality of teacher language use plays a critical role in driving children's early language development. For example, studies have demonstrated that cognitively challenging conversations that address decontextualized or relatively abstract topics are particularly beneficial to children's language development (Dickinson, 2001a, 2001b; Dickinson & Smith, 1994). Unfortunately, not all teachers can provide high quality conversations, comments or questions. This is especially true with those underpaid or poorly prepared teachers serving low-income children in publicly funded programs. A descriptive study within the PCER initiative (Massey, Pence, and Justice, 2008) confirmed these prior findings about teacher talk by examining the quality and quantity of teacher questions in 14 preschool classrooms (both treatment Language-Focused Curriculum and control) serving economically disadvantaged 4-year-olds. They found that questions characterized one third of all teacher utterances, with management questions (e.g., "Are we ready?") occurring most frequently (44.8%), followed by more cognitively challenging questions (32.5%; e.g., "What do you think will happen next?") and less cognitively challenging questions (22.7%; e.g., "What was this called?") That is to say, more cognitively challenging questions represented only one tenth of all teacher utterances in the at-risk preschool classroom. They further examined the frequency of use for different types of questions across various classroom contexts and found that more cognitively challenging questions occurred most frequently in storybook reading. Unfortunately, according to Dickinson (2001a), only 1% to 4% of the total day is typically spent on storybook reading in early care and education settings.
The aforementioned findings naturally raise the following question: Are curricula that extend storybook reading time more effective in promoting children's language development? Several studies within the ISRC and PCER initiatives examined the effects of curricula that include interactive reading activities in the daily plan. The Head Start REDI (Research-Based Developmentally Informed) program developed a curriculum featuring interactive reading activities based on shared reading and dialogic reading, providing teachers with scripted books and targeted vocabulary and instructing teachers to elicit children's language more effectively and to be more responsive. A randomized control trial was employed to compare 4-year-olds in the intervention condition and a similar group in non-intervention Head Start classrooms. The post-intervention tests showed that, after being exposed to the intervention for seven to eight months (September to March/April), children in the treatment group outperformed the comparison group on both vocabulary and language use at home (with effect sizes of .15 and .25, respectively), but no effect emerged on measures of children's grammatical understanding. Similar results were obtained from other curricula that integrated interactive reading activities into the curriculum, such as Children's School Success (CSS) and Literacy Express (LE). HLM analyses demonstrated that CSS improved children's vocabulary and language use at home through changing teacher practice (e.g., more sensitive-responsive talk, richer talk, better instructional support). Teacher practice accounted for 53% and 67% respectively of the intervention effect on vocabulary and language use at home. LE was found to have a significant impact on expressive communication skills (ES = .30) and a potentially positive effect on vocabulary (ES = .45). Over all three measures used in the oral language domain, the average effect size was .36.
In addition, dialogic reading or reading-aloud was an important component of three other early childhood curricula Breakthrough to Literacy (BTL), Ready, Set, Leap! (RSL), and Building Early Language & Literacy (BELL). Despite sharing common purposes, these three curricula differ in activity designs and implementation. Both BTL and RSL are comprehensive language and literacy programs that include activities throughout the day. BTL is built around a series of weekly books with a focus on interactive reading; while RSL utilizes interactive electronic technology and thematically-grouped children's trade books. In contrast, BELL, as an add-on pre-kindergarten literacy program, entails only two daily 15- to 20-minute lessons. The Project Upgrade study compared the curricula to each other and a business-as-usual control group. The results revealed that RSL and BTL had significant impacts on children's definitional vocabulary (ES = .30), even though the impacts were not large enough to reduce the gap (see below). On the other hand, BELL, the less intensive curriculum, yielded no significant impacts on any measures of early language and literacy. Taken together, findings from these studies may suggest that curricula with a focus on interactive reading activities do exert positive impacts on children's oral language development, given enough dosage of implementation.
Even though interactive reading seems to be an effective ingredient to improve oral language, not all curricula put an emphasis on interactive book reading. Instead, some PCER/ISCR curricula provide specific and explicit instructions to teachers to foster frequent and long high quality conversations that use complex syntax and address abstract concepts. For instance, the Language-Focused Curriculum (LFC), designed for preschoolers with language limitations, identified specific linguistic targets (e.g., verb phrase structures, adjective, pronouns, etc.) in daily lesson plans and instructed teachers to use a set of Language Stimulation Techniques (LSTs) to foster the delivery of linguistically-responsive conversations with children. In a study with a random-control trial design, Justice, Mashburn, Pence, Wiggins (under review) analyzed children's 10-minute language samples gathered in the fall and spring, with the amount of professional development that teachers received matched in the intervention group and comparison group. However, they found no impacts of LCF and LST exposure on children's expressive language skills. Instead, the results demonstrated that children who attended preschool more frequently benefited more from the LCF curriculum and LST exposure compared to those with low attendance (no effect size reported). This finding is not unexpected: if a child does not go to class frequently, how can s/he benefit from the curriculum? From another angle, this finding aligns with the result of the Project Upgrade study that showed no impact of the lower dosage curriculum, BELL.
Two other curricula, Let's Begin with the Letter People and Doors to Discovery, also provide teachers with a detailed plan of the scope and activities that are developmentally appropriate to enhance literacy development. This plan provides specific instructions to help teachers determine group size, sequence instructional goals, and match appropriate materials with learning objectives. Both curricula are thematically based and involve the use of learning centers in the classroom. Despite the similarity, Let's Begin with the Letter People has a particularly strong emphasis on letter knowledge and phonological awareness while Doors to Discovery (DD) puts a strong emphasis on language. In an experimental study funded under PCER, Assel and his team (Assel, Landry, Swank, & Cunnewig, 2006) examined the effectiveness of the two curricula across three different settings (Head Start, Title 1, and Universal pre-K classrooms) and included a control group in each setting, in which teachers used teacher-developed, nonspecific curricula. The results revealed that both of the intervention curricula demonstrated similar effectiveness. The auditory comprehension and vocabulary skills of children in classrooms using either of these two curricula grew more than children in control classrooms, but this effect was moderated by program site (Head Start versus Title I versus Universal pre-K). For auditory comprehension, children in Head Start showed the greatest gains compared to children in control classrooms, while for vocabulary, children in Head Start and Title I classrooms showed the greatest gains. Because their primary interest was to identify differences in the rates of growth of child skills over time, the authors acknowledge that their design did not control for differences in children's baseline scores. It was the case that universal pre-K children consistently showed higher initial scores than children in the other two programs, and Title I children outscored Head Start children. Therefore, differences in gains could be due to the fact that the Head Start children, who started with lower baseline scores, had more room to grow.
Two large national evaluations also demonstrated mixed results on children's oral language outcomes. The National Evaluation of Early Reading First, using a regression discontinuity (RD) design, evaluated the effect of additional funding for teacher professional development on teacher, classroom, and child outcomes. A variety of curricula were used in funded and non-funded early childhood sites, however, teachers in the funded sites received more professional development in all areas (language & literacy, assessments, and child development and behavior) than teachers in the non-funded sites. The program demonstrated positive impacts on teachers' language use and book reading practices in the funded classrooms. However, no significant impacts were found on children's oral language skills, as measured by the Expressive One-Word Picture Vocabulary Test or the Auditory Comprehension subscale of the Preschool Language Scale-IV. These findings mirror those from the recent Reading First Interim Study (Gamse, Bloom, Tepper, & Jacob, 2008), which also used an RD design to examine the effects of a federal funding stream at the K-3 level. Although the study found positive effects on teacher instructional practice, those effects did not translate into positive effects in student achievement. On the other hand, the National Head Start Impact Study found small positive impacts on 3-year old children's vocabulary scores (effects sizes in the .10 to .20 range).
Results of the PCER cross-site evaluation, however, were disappointing with respect to oral language. It should be noted that the lack of effects in the PCER cross-site evaluation could be due in part to small sample sizes, to the timing of the baseline testing, which sometimes occurred later than the baseline testing done by the individual evaluations (Assel et al., 2006), or to differences in measures (Justice et al., under review). Only two of twelve curricula were found to have positive impacts on children's oral language skills in either pre-K or kindergarten: DLM Early Childhood Express with Open Court Reading Pre-K (DLM) and the Early Language and Literacy Model (ELLM). For ELLM, effects were found only in kindergarten (not in pre-K), a surprising finding given that 11 of the 14 ELLM teachers were in their second year of implementation of the curriculum at the time of the evaluation. Effect sizes for both curricula were medium and similar in kindergarten for both curricula on the PPVT and the TOLD (Effect sizes range from .34 to .48), while in pre-K, DLM's effects were in the .40 range. One similarity across these curricula is the fact that both ELLM and DLM are implemented in combination with already comprehensive early childhood curricula and provide teachers with ongoing professional development and support, possibly indicating that the amount of curricular support to teachers needs to be fairly substantial in order to obtain effects on children's outcomes.
In sum, these recent federally-funded research initiatives, although far from conclusive, have provided some confirmatory evidence that children's oral language outcomes can be improved when teachers engage in and provide children with more complex language activities and opportunities. The fact that effect sizes for oral language were medium (according to Cohen, 1988), and not small, is a hopeful finding. The positive results, however, may be moderated by numerous factors, including the instructional support for the teacher, the dosage received by the child, and the program site, which in many cases serves as a proxy for other characteristics of children and teachers in those sites, including baseline test scores, poverty status, or teacher experience. Future research should focus on identifying more concretely the factors that need to be put in place to obtain consistent oral language gains, as well as the size of the effect that is needed to ensure success in reading comprehension.
As stated above, the ability to distinguish and manipulate sounds is a strong predictor of reading success. Phonological awareness has been well documented for its critical role in learning to read (e.g., Gunning, 2000; Juel, 1994; Shu, Anderson, & Wu, 2000; Snow, Burns, & Griffin, 1998). Children who are more aware of the different sounds in words and are able to separate or combine sounds are more ready to learn to read and write. Studies have found that explicit instruction in phonological awareness can reduce the incidence of reading failure and thus improve the possibility of reading success (Adams, 1995; Stanovich, 1993; Snow et al., 1998).
In general, less evidence was found that the interventions studied through recent federally-funded research initiatives exerted positive impacts on children's phonological awareness skills than was found in terms of oral language. Neither of the two national evaluations included in this review, of Early Reading First and of Head Start, found effects on children's phonological awareness skills. Similarly, in the PCER cross-site study, 11 of 12 interventions showed no statistically significant effects in this domain (but note that possible limitations for the PCER cross-site evaluation listed above for the oral language domain apply for the phonological awareness domain as well). Only one intervention DLM was found to have positive effects in pre-K and kindergarten as measured by the Pre-CTOPPP (pre-K) or the CTOPP (kindergarten) with effect sizes ranging from .32 to .38.
In contrast to the PCER cross-site evaluation findings, however, the individual evaluations of several curricula indicated some positive effects on children's phonological awareness skills. As mentioned above, differences in findings between the cross-site evaluation and the individual evaluations could be due in part to small sample sizes, differences in the timing of baseline testing, or to differences in measures. For example, Literacy Express was found to have an average positive effect size of .63 in the phonological processing domain, as measured by the P-CTOPPP Blending and Elision subtests at the end of pre-K (Lonigan, 2006). Similarly, the Project Upgrade study demonstrated that Ready, Set, Leap! (RSL) had a significant impact on children's phonological awareness skills at the end of pre-K as measured by the TOPEL (ES = .39, when compared to the control group jointly with another intervention, Breakthrough to Literacy). In a study of Let's Begin with the Letter People and Doors to Discovery (Assel et al., 2006), children in classrooms receiving either curriculum showed greater gains in rhyming skill than those in control classrooms, as measured by the Woodcock-Johnson-3 Sound Awareness subtest (d = .26). Additionally, there were differences in rates of growth by curricula that were moderated by program site, such that universal pre-K classrooms using Let's Begin had higher rates of growth than those using DD by an effect size margin of .85. No differences, however, were found in children's rates of growth between the two curricula in Head Start and Title I classrooms (Assel et al., 2006). The same caveats mentioned above apply to these findings, that is, since Head Start and Title I children began with lower baseline scores than those in Universal pre-K, they may have been more likely to gain at a faster rate .
Some preliminary findings from the ISRC consortium are in line with the aforementioned findings. For example, in the Head Start REDI study, significant impacts on phonological awareness were found (ES =.39 for Blending subtest of the TOPEL, .35 for Elision subtest). This curriculum provided professional development for teachers that focused on implementing sound games (three times per week). The evaluation of Children's School Success found an interaction effect between pretest scores and quality of implementation on children's early literacy outcomes, including phonological awareness. The study found that children who scored lower on pretest measures benefited more from high implementation and less from low implementation of the curriculum.
In sum, recent federally-funded research initiatives have provided mixed evidence of the studied curricula's effectiveness to improve children's phonological sensitivity skills. This lack of consensus could be due to methodological issues such as statistical power or differences in measurement of these skills. Or, it also could be the case that gains in this area are difficult to effect. Future research needs to address these methodological issues so as to produce more conclusive results. In addition, as with oral language, moderating factors such as dosage, children's pre-test scores, and program site are cited in these studies. Planned variation studies would be an important addition to further clarify the role of these moderators of intervention effectiveness.
In line with the core research about the essential role of print and letter knowledge for later literacy success (e.g., Clarke, 1988; Clay, 1991; Torgeson & Davis, 1996; Whitehurst & Lonigan, 2001), the majority of the interventions reviewed targeted children's print knowledge as an essential skill. The goal of these interventions was to improve children's print and letter knowledge skills through training teachers how to a) explicitly teach these skills, and/or b) provide children with opportunities to practice these skills. Was there evidence that the interventions were effective in improving children's print and letter knowledge? Although not entirely consistent, the majority of interventions that targeted this area showed some evidence of positive effects. The national evaluations of ERF (U.S. Department of Education, 2007) and Head Start (U.S. Department of Health & Human Services, 2005) both had positive impacts on children's print knowledge. Head Start reduced, by almost half (47%) the gap in children's ability to recognize letters between Head Start children and the national average for all 3- and 4-year olds. Similarly, the impact of ERF on children's print and letter knowledge was 5.78 standard score points on the Pre-CTOPPP print awareness subtest (ES = .34).
The PCER cross-site evaluation conducted by Preschool Curriculum Evaluation Research Consortium (2008) indicated positive impacts for only two curricula of eleven that focused on children's language and literacy development Curiosity Corner (CC) and DLM. The former curricula had an impact in kindergarten, while the latter had impacts in both pre-K and kindergarten. Of the three measures used, CC demonstrated positive impacts on the TERA and the WJ Letter Word Identification subtest (ES = .43 for both), while DLM had positive impacts on all three measures in pre-K (the TERA, the WJ Letter Word Identification subtest and the Spelling subtest) equaling effect sizes of .68, .51, and .46 respectively. In kindergarten, DLM had impacts only on the TERA and the WJ Letter Word Identification subtest (effect sizes equaled .76 and .50 respectively).
Of the nine remaining curricula that did not demonstrate statistically significant impacts in this domain in the cross-site evaluation, five were studied in individual evaluations and were found to have positive effects (ELLM, Let's Begin, DD, Literacy Express, and Ready, Set, Leap!). For example, the individual evaluation of ELLM suggests that the curriculum, which focuses on instructional strategies and learning materials for teachers to explicitly teach literacy skills and provide structured literacy experiences, had small, positive effects on measures of letter knowledge, print conventions, and meaning of print at the end of prekindergarten in favor of the intervention (effect sizes equaled .25, .28, and .26 respectively). By the end of kindergarten, positive effects were found only on letter knowledge (ES = .34). Similarly, Let's Begin with the Letter People and Doors to Discovery were both found to have positive effects on Head Start children's print knowledge skills, compared to children in Title I or Universal pre-K classrooms (ES = .53 for HS, versus .06 for Title I and .25 for Universal pre-K). The measure used in the study was the WJ-3 Letter Word Identification subtest. In the case of Literacy Express, the curriculum demonstrated statistically significant positive effects on children's skills in this domain, as measured by several assessments - the TERA-3 Alphabet subtest, the TERA-3 Meaning subtest, and the WJ-3 Spelling subtest (effect sizes equaled .57, .83 and .50 respectively). On two other measure - the P-CTOPPP Print Knowledge subtest and the TERA-3 Print Conventions subtest, impacts were not statistically significant, but were large enough by WWC standards to be substantively meaningful (effect sizes equaled .41 and .34 respectively). Finally, in the Project Upgrade study (U.S. Department of Health and Human Services, 2007), RSL, along with BTL had significant impacts on children's print knowledge skills, as measured by the Print Knowledge subtest of the TOPEL (ES = .63).
In summary, the majority of curricula evaluated seem to have been able to exert positive effects in the area of print knowledge across varied assessments and conditions, however there is much more to be done. The more extensive NELP review should provide more insight into common features across interventions that show effects on children's print knowledge. Future research would also benefit from moving beyond establishing the link, as done in the ERF evaluation, that more time spent by teachers on print awareness opportunities is related to children's higher print awareness scores, to identifying more effective ways to teach children alphabetic knowledge. For example, in one non-experimental descriptive study funded by PCER (Justice, Pence, Bowles & Wiggins, 2006), findings based on children in classrooms using either the Language-Focused Curriculum or High/Scope indicated that the order of letter learning was not random and that some letters hold an advantage over others to influence their order of learning. The authors suggest that perhaps early care and education teachers should teach more difficult, less known letters first, since children are more likely to know more common letters. Teachers should also account for individual differences since children know different letters, depending on both extrinsic and intrinsic influences.
A review of some of the interventions evaluated for this review points to the range of activities/components that are often implemented with the goal of producing positive changes in children's early language and literacy outcomes. For example, ELLM includes five components: research- and standards-based literacy curriculum, family involvement, professional development, working partners, and practice-focused research and evaluation. The interrelationships among these components and their interdependence were prominent, and were discussed in almost every study that was reviewed for this paper. When these comprehensive curricular approaches are implemented in early childhood settings, which are dynamic and complex learning environments in themselves, it becomes difficult to tease out the critical features for success from the wide range of possible influences. Yet, is important to understand what factors might be moderating the effectiveness of interventions. Because variation in these factors was not a focus of this body of research the aim of which was to provide evidence of effectiveness of the interventions studied, on average researchers were not always able to address questions about moderating factors. In addition, most analyses of moderators were conducted outside an otherwise experimental design, and as such, cannot be considered causal. Despite these limitations, in the research reviewed in this paper, some, mostly non-experimental evidence was provided regarding three possible critical factors that the studies suggest may be important moderators of intervention effectiveness: professional development, dosage of implementation, and child background characteristics.
Before implementing the specific curriculum, teachers (and sometimes other educational staff) usually received professional development or training on how to deliver the intervention. Some interventions also provided ongoing coaching to monitor or refresh ideas and to solve problems rising during ongoing implementation. Professional development may affect the impact of an intervention through changing teachers' practice and fidelity. Using non-experimental methods, the LFC study showed that treatment teachers exhibited strikingly high fidelity to the curriculum immediately following a professional development workshop (Pence, Justice, & Wiggins, in press). This aligns with the findings of an evaluation of Building Language and Literacy in Montgomery County public schools (Ramey, Ramey, Kleinman, Lee, Farneti, Timraz, Nielsen, et al., 2008 unpublished manuscript), which compared two coaching conditions: weekly versus monthly. It revealed that weekly work-embedded coaching significantly improved implementation levels of the curriculum and yielded significant positive impact on children's literacy skills (ES = .44). These contrasts were tested within the experimental design and indicate that sufficient professional development may be related to the success of an intervention.
Professional development can even compensate for the insufficiency in teachers' educational background. The Project Upgrade study (U.S. Department of Health and Human Services, 2007) analyzed (outside of the experimental design) the observational data from study classrooms, and, surprisingly, instead of finding an educational background effect, the results demonstrated that the interventions eliminated the differences between better-educated teachers and less-educated teachers. Teachers in the treatment group all looked remarkably similar, regardless of their educational levels, compared with the dramatic differences among control group teachers. In other words, the professional development that treatment group teachers received and the well-specified curricula diminished the differences in teaching instruction due to teacher educational background. Similarly, another group of researchers (Lieber, Goodman-Jansen, Horn, Palmer, Hanson, Czaja, Butera, et al., 2007) examined 30 Head Start teachers in implementing the CSS curriculum and found that coaching and teachers' motivation to change, rather than teaching experience or degree status, affected curriculum implementation. These analyses were correlational, and outside of the experimental framework.
Dosage of Implementation
Program dosage can be measured in days of children's attendance during the academic year. When measured in this way, greater program dosage has been found to be related to stronger program impact. For example, in a study of two state public pre-K programs, Ramey, Ramey, and Stokes (year not provided) found that children who received the full day and full school year LA4 program (Louisiana) gained nearly twice as much from the program as their peers who received only the half-year LA4 program (pilot year) or the half-day full year Montgomery County Public Schools program (Maryland). Similarly, in an experimental study of LFC, researchers found that children who attended early care and education regularly benefited more from the intervention than those with low attendance rates (Justice, Mashburn, Pence, Wiggins, under review). It can be inferred that children who attended school more regularly were exposed to a higher dosage of the intervention compared to those who attended school less regularly.
Program dosage can also be thought of in terms of the amount of time that has been allotted for the curriculum to be implemented. In the Project Upgrade study (U.S. Department of Health and Human Services, 2007), the three curricula being compared RSL, BELL, and BTL all focused on the development of early literacy skills and knowledge. However, they were distinguished from one another in terms of instructional approach, materials provided, intensity and cost. Both RSL and BTL are full-day comprehensive curricula; BELL is an add-on literacy program entailing only two 15-20 min sessions daily. The finding that both RSL and BTL had significant impacts on all literacy measures compared to the lack of impacts of BELL suggests that dosage of the intervention should account for part of the differences in impacts on children's outcomes. This is more persuasive considering that BELL had a much stronger focus on phonological awareness than the other two curricula, yet had no impact on children's phonological awareness while RSL and BTL did.
Child Background Characteristics
Findings of some studies also revealed that child background characteristics (such as family economic status, pretest performance, personality, and language ability) may moderate the impacts of interventions. The research demonstrated that the interventions were effective for all children, but were particularly effective for some children. For example, children who were more economically or academically disadvantaged were found to have gained more from interventions than their more advantaged peers (Assel et al, 2006; Ramey, Ramey, & Stokes, 2008 unpublished manuscript; Odom, Diamond, Hanson, et al., 2007). In a study that examined the contributions of child characteristics to the quality of teacher-child relationship, Rudasill, Rimm-Kaufman, Justice and Pence (2006), in their study of LFC, demonstrated that individual differences in child temperament and language skills affected teacher-child interactions, which ultimately contributed to intervention effect. This was especially true for early language and literacy curricula, in which teacher-child conversations are often key cornerstones of the implementation.
English Language Learner (ELL) status is also a very important factor when considering children's early language and literacy skills. Although most studies included ELLs in their study samples, results were not often reported by subgroup. This was perhaps due to power issues, since most studies were not powered to detect subgroup differences. An exception is the LA ExCELS (Los Angeles: Exploring Children's Early Learning Settings) study which explored ELL children' experience in early care and education settings and their school readiness outcomes (Fuligni, 2008). Preliminary results showed that low income bilingual Spanish children were behind monolingual peers in several language and school readiness domains during pre-kindergarten period. There were no differences in experiences of Spanish speaking and English monolingual children in early care and education programs at age 4. However, participation in early learning settings was particularly beneficial for Spanish speaking children. This is consistent with the aforementioned pattern that academically disadvantaged children benefited more from interventions or programs compared to their advantaged peers.
Child background in terms of family factors also includes family literacy environment and parent behavior. These issues were addressed by the Getting Ready Nebraska program. In this program, several studies examined the effects of home literacy and parents' belief or behavior on children's development. In a study investigating adolescent parents' participation in learning and their perceptions of professional support, Knoche, Woods, & Sheridan (2008) found that for children whose parents demonstrated low levels of parent learning behaviors, high levels of professional support were associated with higher scores in young children's language skills.
In sum, the federally-funded evaluation studies reviewed here provide support and replicate previous findings about factors that may be important as moderators of intervention effectiveness. However, many questions remain. For example, how much professional development is optimal? What amount of dosage of intervention is needed for children to progress? What interventions work best for which children? One way to address this in the future would be to conduct planned variation studies, in which hypotheses about "how much" and "for whom" can be tested. From this data, threshold levels for professional development and dosage, for example, could be more clearly understood and ultimately be used to inform intervention developers and policy makers.
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