Limitations of the Flanagan et al. (2006) Synthesis
Although the sheer number of key, review, and individual studies populating the Flanagan et al. (2006) research syntheses looks impressive, the syntheses suffer from four primary limitations.
First, it would impossible to replicate the CHC COG-ACH summaries of Flanagan et al. (2006) due to the lack of: (a) specified and published database search terms and date ranges, (b) significant CHC- ACH relation significance criteria, (c) study inclusion and exclusion criteria, and (d) procedures to control for possible publication bias (see Cooper, 1998; also see Pillemer & Light, 1980). Second, the use of the broad dependent variable (DV) domains of reading, math, and writing make it impossible to determine if the salient COG-ACH relations generalize across sub-achievement domains (e.g., basic reading skills vs. reading comprehension). Third, the select and limited developmental CHC COG-ACH trends discussed by Flanagan et al. (2006) are based only on a handful of key CHC studies. The age characteristics of the bulk of the individual studies included in the Flanagan et al. (2006) research summaries are ignored (i.e., no attempt was made to present CHC COG-ACH results by age- differentiated sub-samples).
Finally, specification error, sometimes called omitted-variable bias (Keith, 2006), occurs when potentially important variables in predictive or explanatory research are not included in a study’s research design. Specification error can lead to biased estimates of the effects (i.e., relative importance) of individual predictor variables on the dependent variable of interest (Pedhazur, 1997). Almost all the reviewand individual studies listed by Flanagan et al. (2006) likely suffer from an unknown degree of specification error since the intent of their synthesis was to identify the relative importance of the various CHC domains within the CHC framework. One example is presented below.
Floyd et al. (2003) used an analogy from sports to explain specification error. 
"As an analogy, a college basketball coach could construct a regression model that included season-long player statistics of starters in order to predict post-season performance. If the coach omitted the game statistics for the point guards and included only the statistics for the remaining four starters, over- or under- estimates of the importance of one or more of the other four starting positions would probably result. These biased findings might lead the coach to make erroneous decisions about the strengths of certain player positions when developing game strategies. Because of the failure to include measures of potentially important." (p. 156).
Flanagan et al. (2006) list the individual study of Wagner et al. (1997) as evidence for the importance of Gc, Ga, and Glr in reading. Specification error is present in this study due to the lack of indicators of the more complete array of important predictor CHC abilities for reading. The CHC cognitive abilities of Ga, Gc, and Glr may have displayed slightly different degrees of relative importance (and might have demonstrated non-significance) had Wagner et al. (1997) included measures of Gsm, Gf, Gs, and Gv in their predictor models. 
In the Flanagan et al. (2006) summary tables, Aaron (1995)  is classified as a review study. Inspection of the original article finds it not to be a systematic review in the traditional sense of aliterature review  (Cooper, 1998). Aaron's article describes a diagnostic procedure for reading disabilities and draws upon select literature sources.  This is not a criticism of the original purpose of the Aaron (1995) publication, nor is this a criticism of the categorization of the Aaron study as a review in the published summary tables. The point is that the term review typically has the connotation in the scientific literature of systematic narrative or empirical research synthesis of extant research studies.