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Research Article: Systematic Reviewand Meta-Analysis

Comparing the Overall Result and Interaction in Aggregate Data Meta-Analysis and Individual Patient Data Meta-Analysis

Huang, Yafang MD, PhD; Tang, Jinling MD, PhD; Tam, Wilson Wai-san PhD; Mao, Chen MD, PhD; Yuan, Jinqiu MD, Msc; Di, Mengyang MD, PhD; Yang, Zuyao MD, PhD

Section Editor(s): Paraskevis., Dimitrios

Author Information
doi: 10.1097/MD.0000000000003312



Meta-analysis is a statistical method that integrates results from different studies to produce an overall result. Clinicians, guideline developers, and policymakers increasingly use meta-analyses as high-quality evidence aids to make decisions and recommendations.1–3 Meta-analysis can either be based on aggregate data or on individual patient data.4,5 Aggregate data meta-analysis (ADMA) combines the grouped data of primary studies, whereas individual patient data meta-analysis (IPDMA) synthesizes the individual data of primary studies.6,7 In principle, IPDMAs have advantages over ADMAs, as they can more strictly standardize included studies in factors such as patient characteristics, treatment details, and duration of follow-up; they can also more effectively conduct subgroup analyses and control confounding in such analyses.8 Thus, IPDMA is often viewed as superior to ADMA and can produce higher-quality evidence.4,6,9 However, ADMAs are much more common because they are much more rapid and require far less resources to produce.10,11 A study showed that >95% of published meta-analyses were ADMAs.12 Several studies have compared ADMAs with IPDMAs.13–18 These studies include a very small number of meta-analyses and focused only on the overall result.

We conducted this comprehensive review to compare a previous ADMA with its subsequent IPDMA of the same topics to investigate how often ADMAs agree with IPDMAs, what factors affect the agreement, and how effective IPDMAs are in exploring interactions compared with ADMAs.


Literature Search Strategies

A literature search on IPDMA articles was conducted in August 2012. A total of 829 eligible IPDMA articles were identified. Details have been reported elsewhere.19 Briefly, all IPDMA articles were identified by a comprehensive search of PubMed, EMBASE and the Cochrane Library with an established search strategy. For each of the 829 IPDMA articles, PubMed was searched to find matching ADMAs against the disease and intervention of the IPDMA. A total of 829 searches were conducted. The search was further limited by using Montori's balanced 5 search terms for identifying systematic reviews.20 Details of the search strategy can be found in Supplemental Content, We also scrutinized the references of each eligible IPDMA article. An updated search was conducted in March 2013. Results of the PubMed search for each IPDMA article were saved in EndNote libraries separately.

Selection of ADMA Articles

The screening of matched ADMA articles was carried out separately for each eligible IPDMA by using the results saved in the EndNote library. ADMA articles that were the same as or similar to the IPDMA with regard to patients, test intervention, control intervention, and at least 1 outcome were selected.21 We then excluded ADMA articles that were published after the index IPDMA article. If >2 ADMA articles were found for 1 index IPDMA, the ADMA that was published immediately before the IPDMA was considered eligible and used in the final analysis.

We excluded articles published in non-English languages, qualitative reviews without meta-analysis, and IPDMA articles. ADMA articles on diagnostic accuracy and matched ADMAs that did not report the overall combined result were also excluded. If ≥2 pairs of reviews were found on the same topic, we used the most up-to-date pair.

Two authors independently screened the titles and abstracts. They subsequently screened the full-text articles for which eligibility remained unclear. Any discrepancies were resolved by consensus or by consulting a third author if the 2 failed to reach an agreement.

Data Extraction

For each matched pair of ADMA and IPDMA article, ≥1 matched meta-analyses could be eligible and were all extracted. All the matched meta-analyses extracted must be matched by patient, intervention, comparator, and outcome.22 The matched meta-analyses formed the basic data for our analysis.

We extracted information from each article on disease, test intervention, control intervention and outcome, direction of effect, statistical significance of the estimate, and number and significance of subgroup analyses and of interaction terms. The direction of effect was divided into 2 groups: the test treatment is more effective than the control treatment and the test treatment is equally or less effective than the control.

A subgroup analysis refers to an analysis in which the trials or patients are divided into subgroups according to an attribute of the trial or patient, and the results are combined in each subgroup and then compared among subgroups. The attribute can be treatment dosage, treatment in the control group, patient characteristics, treatment setting, and so on. The third factor beyond the treatment and outcome can be potential effect modifiers, for example, patients’ demographic factors and lifestyle of patients, as well as co-morbidities or characteristics of the disease.23 Subgroup analyses according to these factors will be considered to assess interaction or effect modifications. A product term between the test treatment and a third factor in a regression analysis was also considered an analysis for interaction. Only subgroup analyses conducted in the matched meta-analyses were extracted.

Statistical Analyses

Descriptive analyses were conducted to summarize the characteristics of the included meta-analyses. Percentage was used for categorical variables, and median and interquartile range (IQR) were used for continuous variables. The Wilcoxon signed-rank test was used to detect the differences in number of studies, patients, and length of follow-up between ADMAs and IPDMAs. The number of meta-analyses that conducted subgroup analyses was analyzed by McNemar χ2 test.

We used the methods of Villar et al24 to define the agreement and disagreement between the paired ADMA and IPDMA. An ADMA was classified as being in agreement with its matched IPDMA if the effect of both was in the same direction. Otherwise, they were classified as being in disagreement.

We investigated the association of agreement with the following characteristics of the ADMAs: research topic (treatment or prognosis), types of outcome (objective or subjective), study design (randomized controlled trials or others), search for grey literature (yes or no), request for data from author (yes or no), use of intention-to-treat analysis (yes or no), significance of testing results (significant or nonsignificant), direction of effect of the test intervention compared with the control (greater benefit or equal benefit/greater harm), and between-study heterogeneity (yes or no). The total and percentage of significant subgroup analyses and interactions between ADMAs and IPDMAs were also compared. χ2 test or Fisher exact test (when the expected cell frequency is <5) was used for comparison.

In a sensitivity analysis, we extracted the original aggregate data, which were available in a fraction of the ADMAs, and re-estimated the overall result by using the same effect measure used in the matched IPDMA. This step allowed us to directly compare the size of effect between the ADMA and the IPDMA and estimate the agreement differently. If no statistically significant difference was found between the matched ADMA and IPDMA, we assumed consistency in their overall result. Otherwise, inconsistency was assumed.25 This is a commonly used method to quantify the agreement between meta-analyses, but it requires the estimation of the effect with the same effect measure.25 We used SPSS (version 18.0 for Windows, SPSS Inc, Chicago, IL) to perform the analyses and used R 3.2.3 to plot the figure.


From the 829 IPDMA articles, 71,522 citations were identified from the PubMed search and references of the IPDMA articles. A total of 129 ADMA matched articles were found, which resulted in 204 matched meta-analyses eligible for this study. Figure 1 shows the details of the search for matched ADMAs and the results of each search step.

Flowchart of the literature search for prior paired ADMA articles. *Articles with a smaller PubMed ID number are published earlier than those with a larger PubMed ID number. ADMA = aggregate data meta-analysis, IPDMAs = individual patient data meta-analysis.

The characteristics of the 204 matched meta-analyses are summarized in Table 1. Of the 204 matched meta-analyses, 69 (33.8%) studied cardiac and cardiovascular diseases, 132 (64.7%) were on drugs or biologics, 43 (21.1%) used placebo as control, and 187 (91.7%) used objective outcomes. A total of 66.2% (135/204) of ADMAs and 66.7% (136/204) of IPDMAs showed that the test treatment was better than the control. In comparison, 61.3% (125/204) of ADMAs and 63.7% (130/204) of IPDMAs showed significant overall results.

Characteristics of the Included 204 Matched Meta-analyses

In total, 187 (91.7%) of the 204 matched ADMAs and IPDMAs were in agreement, which was an effect in the same direction (Table 2). The agreement is even higher if grey literature was sought (P = 0.025), data from authors were requested (P = 0.012), intention-to-treat analysis was used (P = 0.027), and the overall result was statistically significant (P = 0.001). The remaining characteristics evaluated did not seem to significantly affect the agreement; the characteristics include research topic, type of outcome, study design, direction of effect, and heterogeneity. The consistency rate was slightly lower than the agreement rate and was affected by research topic and study design (Table 2). Figure 2 presents the effect sizes and 95% confidence intervals (CIs) of the matched IPDMAs and ADMAs.

Agreement Rate and Consistency in the 204 Paired ADMAs and IPDMAs According to the Characteristics of the Prior ADMAs
Comparisons of IPDMAs with ADMAs. Note: Effect refers to log-OR, log-RR or mean difference. Bars indicate mean difference (95% CI), log RR (95% CI), or log OR (95% CI). The mean differences of 2 studies (19 and 20) were rescaled by dividing 10 in the plot. The solid horizontal line in the middle represents no effect. ADMA = aggregate data meta-analysis, CI = confidence interval, IPDMAs = individual patient data meta-analysis, OR = odds ratio.

Fifty-three (26.0%) ADMAs and 121 (59.3%) IPDMAs reported subgroup analyses, suggesting IPDMAs are twice as likely to report subgroup analyses as their matched ADMAs. The number of subgroup analyses reported is 150 of the 204 ADMAs and 634 of the 204 IPDMAs (Table 3), which resulted in 8 (5.3%) and 55 (8.7%) statistically significant results, respectively.

Number (%) of Subgroup Analyses and Interaction Terms in 204 Paired ADMAs and IPDMAs According to Reporting and Result of the Significance Testing

Not all the subgroup analyses were on interaction; for example, some were on a dose–response relationship and some on methodological quality. The number of subgroup analyses on interaction was 68 (45.3%) in the ADMAs and 544 (85.8%) in the IPDMAs. The IPDMAs reported 7 times more subgroup analyses on interaction than their matched ADMAs. The number of statistically significant interactions reported is 3 (4.4%) in ADMAs and 44 (8.1%) in IPDMAs. The IPDMAs reported 14 times more statistically significant interactions than their matched ADMAs (Table 3).

In addition, of the 634 subgroup analyses in IPDMAs, 215 (33.9%) studied demographic factors and lifestyle, 202 (31.9%) comorbidities, and 127 (20.0%) studied characteristics of the disease. In comparison, of the 150 subgroup analyses in ADMAs, 23 (15.3%) studied demographic factors and lifestyle, 16 (10.7%) comorbidities, and 29 studied (19.3%) characteristics of the disease.


IPDMA is generally considered scientifically more rigorous than ADMA. The high agreement rate between the matched ADMAs and IPDMAs implies that ADMAs can provide valid conclusions most of the time. The agreement rate can be improved to >95% if the ADMA can improve methodologically in a number of aspects, such as requesting necessary data from authors, searching for grey literature, and using intention-to-treat analysis. We included all the meta-analyses regardless of the fields of study and a largest number of meta-analyses in various fields. Our study may provide a much more generalizable comparison between ADMAs and IPDMAs.

The 2 types of meta-analyses differed greatly in subgroup analyses and interactions found. In particular, the IPDMAs reported 3 times more subgroup analyses and 7 times more subgroup analyses on interaction than the ADMAs, although the percentage of subgroup analyses that were statistically significant did not differ between the 2 types of meta-analyses. More importantly, the IPDMAs found 14 times more interactions that were statistically significant than the matched ADMAs. As confounding can be more effectively controlled in subgroup analyses in IPDMAs than in ADMAs, IPDMAs are also more likely to provide valid conclusions on interaction, although this possibility was not explored in this investigation.

Of the 17 ADMAs that disagreed with IPDMAs on the combined results, 8 cases were the ADMAs with positive result (intervention was superior), whereas IPDMAs were equivalent or inferior to the control. This disagreement may partly be explained by a possible selection bias in the ADMAs.26 Many studies are published in non-English languages or in a conference abstract, journal correspondence, and book chapter.27 These publications are sometimes called “grey literature” and often report a negative and/or nonsignificant result. Failure to include grey literature searches would thus cause selection bias and lead to overestimating of the true effect. Indeed, 6 of these 8 ADMAs (ie, 75%) did not search the grey literature.

Conversely, of the 17 ADMAs that disagreed with IPDMAs on the combined results, 9 cases were ADMAs with a negative result (intervention equivalent or inferior to the control), whereas the IPDMAs were estimating that intervention was superior. This may partly be explained by possible information bias in the ADMAs. Often, information bias in meta-analyses arises in the form of “data availability bias,” in which an ADMA is based on incomplete data. It usually happens when data are not openly reported in the original publications and the authors of ADMAs do not have access to the full set of data if they do not contact or fail to receive a reply from the authors of the original studies. Indeed, 8 of the 9 ADMAs (ie, 89%) did not contact the authors for the original studies. The median number of patients is 1086 in the 9 ADMAs, which is only half of the number of patients (2014) included in their matched IPDMAs.

Knowing whether the effect of a treatment differs according to demographic factors and lifestyle, comorbidities and characteristics of the disease are important in making better decisions. The IPDMAs reported many times more subgroup analyses according to these factors than the ADMAs. One reason the ADMA is not capable of assessing interactions is that corresponding subgroup analyses are not conducted and reported in the original studies. Subgroup analyses in trials are generally not encouraged and should be conducted only with the right reasons, to prevent false-positive results. Another reason is that original trials performed subgroup analyses but grouped the same variable in different ways, which makes it difficult and less meaningful to combine the subgroup analyses. This problem can be easily overcome in an IPDMA that has individual data.

No significant difference was found in the proportion of significant subgroup analyses and significant interactions between the matched ADMAs and IPDMAs. If subgroup analyses in the ADMAs are assumed to be based on cautiously planned subgroup analyses in original trials, this finding would suggest that subgroup analyses in IPDMAs do not seem to have resulted in many false-positive findings.

When discussing the implications of the results of this study, one important issue must be revisited: When is the evidence on the effectiveness of a treatment sufficient to justify a recommendation? An ADMA has >90% chance of agreeing with the IPDMA. Is this good enough? Is the expensive and time-consuming IPDMA still necessary in such a case? The answer probably depends on the context. The best is the enemy of the good. If we wait for the best, we may never be able to act because definitive evidence can rarely or never be reached with regard to the effectiveness of medical interventions.28 In fact, about 50% of widely used medical interventions are of an uncertain effect.28 We believe that 90% certainty is good enough for action in many medical circumstances. This would be particularly true when the treatment is safe and cheap and the potential benefit from the treatment is large. Although further studies may be published in the future, it is unlikely that any new studies would change the conclusion of this research.

Subgroup analyses provide important information for decision-making, as it can help make more relevant and precise decisions. However, subgroup analyses in an individual trial are much less reliable. Owing to this aspect, IPDMA is much more powerful than the ADMA. This special contribution seems to make the IPDMA an indispensable tool in summing up evidence, which will be particularly true if the effectiveness of a treatment is found to be highly heterogeneous.

Although the IPDMA is superior for conducting subgroup analyses, it is unlikely that the IPDMA can be conducted for all topics in the future. Thus, in the many years or even decades to come, our traditional thinking on the analysis and reporting of clinical trials may change, and conducting and reporting of subgroup analyses on factors such as age, sex, ethnicity, comorbidities, and disease severity must be encouraged as much as possible. Given the availability of web-based publications, any subgroup analysis can be reported, though clinical action should normally wait for the combined result of a meta-analysis. In addition, standardizing the way certain variables (eg, age) are classified would be useful so that meta-analyses can easily combine these variables.


Compared with IPDMAs, ADMAs appear to be able to provide a valid conclusion regarding the overall result in most circumstances and can be further enhanced by improving the methods of the ADMA. However, the IPDMA has clear advantages over the ADMA in subgroup analyses and in identifying interactions. Given that conducting IPDMA for all topics is unlikely, encouraging original studies to conduct and report more subgroup analyses is important so that they can be combined in future meta-analyses.


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