 Research article
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Estimating required information size by quantifying diversity in randomeffects model metaanalyses
BMC Medical Research Methodology volume 9, Article number: 86 (2009)
Abstract
Background
There is increasing awareness that metaanalyses require a sufficiently large information size to detect or reject an anticipated intervention effect. The required information size in a metaanalysis may be calculated from an anticipated a priori intervention effect or from an intervention effect suggested by trials with lowrisk of bias.
Methods
Information size calculations need to consider the total model variance in a metaanalysis to control type I and type II errors. Here, we derive an adjusting factor for the required information size under any randomeffects model metaanalysis.
Results
We devise a measure of diversity (D ^{2}) in a metaanalysis, which is the relative variance reduction when the metaanalysis model is changed from a randomeffects into a fixedeffect model. D ^{2} is the percentage that the betweentrial variability constitutes of the sum of the betweentrial variability and a sampling error estimate considering the required information size. D ^{2} is different from the intuitively obvious adjusting factor based on the common quantification of heterogeneity, the inconsistency (I ^{2}), which may underestimate the required information size. Thus, D ^{2} and I ^{2} are compared and interpreted using several simulations and clinical examples. In addition we show mathematically that diversity is equal to or greater than inconsistency, that is D ^{2} ≥ I ^{2}, for all metaanalyses.
Conclusion
We conclude that D ^{2} seems a better alternative than I ^{2} to consider model variation in any randomeffects metaanalysis despite the choice of the between trial variance estimator that constitutes the model. Furthermore, D ^{2} can readily adjust the required information size in any randomeffects model metaanalysis.
Background
Outcome measures in a single randomised trial or a metaanalysis of several randomised trials are typically dichotomous, especially for important clinical outcomes such as death, acute myocardial infarction, etc. Although metaanalysts cannot directly influence the number of participants in a metaanalysis like trialists conducting a single trial, the assessment of the metaanalytic result depends heavily on the amount of information provided. A limited number of events from a few small trials and the associated random error may be underrecognised sources of spurious findings. If a metaanalysis is conducted before reaching a required information size (i.e., the required number of participants in a metaanalysis) it should be evaluated according to the increased risk that the result may represent a chance finding. It has recently been suggested that sample size estimation in a single trial may be less important in the era of systematic review and metaanalysis [1]. Therefore, the reliability of a conclusion drawn from a metaanalysis, despite standardly calculated confidence limits, may depend even more on the number of events and the total number of participants included than hitherto perceived [2–8]. Both numbers determine the amount of available information in a metaanalysis. The information size (IS) required for a reliable and conclusive metaanalysis may be assumed to be at least as large as the sample size (SS) of a single wellpowered randomised clinical trial to detect or reject an anticipated intervention effect [2–4].
The estimation of a required information size for a metaanalysis in order to detect or reject an anticipated intervention effect on a binary outcome measure should be considered based on reasonable assumptions. These assumptions may be derived from two kinds of information. Firstly, by anticipating an a priori intervention effect, most appropriately decided at the time when the protocol for a systematic review is prepared. An a priori intervention effect may be estimated by consulting related interventions for the same disease or the same intervention for related diseases suggesting a clinically relevant effect to be detected or ruled out [2–4]. This situation would be almost analogous to the hypothesis testing in a single randomised trial. Secondly, an intervention effect estimated by trials with lowrisk of bias in the metaanalysis may represent our best estimate, at a given time point, of a possible intervention effect knowing the available data [5]. This would be a kind of a post hoc analysis of the information needed to detect or reject an intervention effect suggested by data already available. When planning a new trial it may be very important to estimate which IS is needed for the updated metaanalysis to be conclusive. In both instances the estimated required information size may be applied to grade the evidence reported in a cumulative metaanalysis adjusting for the risk of random error due to repetitive testing on accumulating data [5, 6]. If the number of actually accrued participants falls short of the required IS the metaanalysis may be inconclusive even though the confidence interval is suggestive of a clinical relevant effect or. Because if the confidence interval (or the pvalue) is appropriately adjusted with sequential methods, it may no longer show a statistically significant or clinically relevant effect. Conversely, if the actually accrued number of participants supersedes the required information size without the metaanalysis becoming statistically significant we may be able to rule out the anticipated intervention effect size [5].
It is not realistic to assume that the population of the included trials in a metaanalysis is truly homogenous, as it may be in a single clinical trial. Metaanalysis, therefore, should not analyse included participants as if they are coming from one trial [9]. Consequently the difference between obtaining the required IS and SS is rooted in the underlying assumption of between trial variability, and thus, the chosen metaanalytical model.
If the betweentrial variability of the outcome measure estimates in a metaanalysis is incorporated into the model using the traditional oneway randomeffects model, the required IS will be affected [5]. In this vein, the required IS is a monotonically increasing function of the total variability among the included trials. An estimate of the required IS can therefore be derived once the degree of variability is known or prespecified [5]. The test statistic for heterogeneity in a metaanalysis, the inconsistency factor (I ^{2}) based on Cochran's Q proposed by Higgins and Thompson [10], may seem an obvious quantity to use for this purpose as it allow us to estimate the degree of the variation, which is not covered by assumption of homogeneity [5]. However, I ^{2} is derived using a set of general assumptions that may be inappropriate in this context.
In this paper we derive a general expression for the required IS in any randomeffects model. We prove the monotone relationship between IS and the degree of total variability in a oneway randomeffects metaanalysis. We use our results to define a quantification of diversity (D ^{2}) between included trials in a metaanalysis, which is the relative model variance reduction when the model of pooling is changed from a randomeffects model into a fixedeffect model. We analyse and discuss the differences between our definition of diversity, D ^{2}, and the commonly used measure for heterogeneity, I ^{2}.
Methods
2.1 Deriving the required metaanalysis information size and diversity
If the required IS needed to detect or reject an intervention effect in a metaanalysis should be at least the sample size needed to detect or reject a similar effect in a single trial, then the following scenario applies:
Let μ _{F} denote the weighted mean intervention effect to be detected in a fixedeffect model metaanalysis and let μ _{R} denote the weighted mean intervention effect to be detected in a in a randomeffects model metaanalysis using generic inverse variance weighting. The information size (N _{F}) needed to reject an intervention effect μ _{F} in the fixedeffect model (with a type I error less than α, a type II error less than β, and equal group sizes) becomes [11, 12]:
and the information size (N _{R}) needed to reject μ _{R} in the randomeffects model (with a type I error less than α, a type II error less than β, and equal group sizes) becomes [11, 12]:
Where and are the variances in the two models with w _{ i }and being the weights in the fixed and randomeffects model respectively. The ratio of information sizes needed in the two models may be calculated as:
under the assumption that μ _{F} = μ _{R} it follows that:
or
the relationship between the IS (N _{ R }) for a randomeffects model and the SS (N _{ F }) for a fixedeffect model is therefore multiplicative by an adjustment factor A _{ RF }
Let τ ^{2} denote the betweentrial variance, k the number of trials, and the 'typical' momentbased sampling error within the trials according to Higgins and Thompson [10], then:
and combining 2.6 and 2.7:
This yields the intuitive interpretation that the required IS in a randomeffects model is a monotone increasing function of the degree of heterogeneity.
2.2 Limitations of a momentbased 'sampling error' in the definition of heterogeneity, I^{2}
Higgins and Thompson [10] analysed candidate measures of intertrial variability and decided on the inconsistency factor I ^{2} under the assumption that all weights w _{i} were approximately equal, that is, for all k trials, that is: ∀i ∈ {1,..., k} is . However, this assumption may not be met in many metaanalyses. In the attempt to generalise the use of I ^{2} to the situation with trial weights being unequal a 'typical' sampling error σ ^{2} of the included trials is assumed [10]. To use a 'typical' sampling error σ ^{2} may not be appropriate in some metaanalyses as weight percentages of the trials easily range from 0.5% to 50% or wider without a known distribution (Table 1 and Table 2). The attempt to estimate the trials' 'typical' sampling error as a momentbased sampling error [10] may be misleading as it attributes less emphasis to the trials with a very high number of participants and events. I ^{2} is interpreted as and intends to measure the percentage of total metaanalysis variability explained by betweentrial variation. In this vein,I ^{2} has been interpreted as the between trial variance relative to the sum of the betweentrial variance and a 'typical' momentbased sampling error or as it has been phrased: "the betweentrial variance rather than the sampling error"[10]. I ^{2} achieves some of the desired properties to characterise betweentrial variability. However, the concept of a 'typical' sampling error is not relevant if it provides a misleading estimate, seriously distorting the I ^{2}calculation or interpretation. If overestimates the sampling error then I ^{2} will be underestimated and vice versa. In such instances it may in fact be wise to abandon the concept of a 'typical' sampling error.
If the focus is shifted towards a sufficient IS estimation, then adjusting factors based on I ^{2} calculated from a momentbased sampling error may be insufficient. We therefore suggest to consider an alternative adjusting factor to obtain an adequate estimation of the required IS.
2.3 Defining and implementing a measure of diversity
Assume we are interested in showing or rejecting a significant intervention effect, μ, regardless of the choice of metaanalysis model (fixed or random). That is, assume μ = μ _{F} = μ _{R}. We then define diversity (D ^{2}) as the quantity compelled to satisfy the following equation:
Solving the equation with respect to D ^{2} we get the definition of D ^{2} explicitly:
As long as we do not know what the difference between I ^{2} and D ^{2} covers, knowing now from 2.9 that D ^{2} reflects the total relative variance expansion changing from a fixedeffect into a randomeffects model metaanalysis, we find it wise to denote D ^{2} diversity instead of just another calculation of heterogeneity. A _{ RF }will be an adjustment of N _{ F }to N _{ R }taking into account the total variance expansion changing from a fixedeffect into a randomeffects model. Hereby, D ^{2} expresses the relative variance reduction when the model of metaanalysis is changed from a randomeffects model into a fixedeffect model. D ^{2} is the percentage of change in variance when the model is changed. D ^{2} becomes exactly the proportion that the betweentrial variance component (τ ^{2} = k·(V _{ R } V _{ F })) constitutes of the sum of variances () in the variance component model if and only if (a sampling error originating from diversity or the required information size) is defined as:
Diversity can then be expressed as:
This way, D ^{2} in a metaanalysis may become a central measure of the betweentrial variability relative to the sum of the betweentrial variability with an estimate of the sampling error basically originating from the required information size.
As such, D ^{2} is able to quantify the relative model variance change from a randomeffects into a fixedeffect model. More importantly D ^{2}, in contrast to I ^{2}, is not based on underlying assumptions of a 'typical' sampling error that are violated in most metaanalyses. D ^{2} is the percentage of the total variance (the sum of between trial variance and sampling error), in a randomeffects model, contributed by the between trial variance.
2.4 Simulating metaanalyses
In our simulations, we considered metaanalyses with k = 6 and k = 20 trials. For each k, we considered the four combinations from two different average control event proportions, (PC) of 10% and 30%, and two true values of the overall effect in terms of odds ratios of 1 and 0.7. The above values were selected aiming to cover different plausible metaanalytic scenarios. In total, these values make up for 8 simulation scenarios.
For each combination of the above mentioned variables we generated data for k 2×2 tables. For all k trials, within group sample sizes were determined by sampling an integer between 20 and 500 participants. Group sizes were equal in each simulated trial. We drew the trial specific control group event rate, PC _{ i }, from a uniform distribution, PC _{ i }~U(PC0.15, PC+0.15). We drew the number of observed events in the control group from a binomial distribution e _{ iC } ~bin(n _{ i }, PC _{ i }). For each metaanalysis scenario we varied the degree of heterogeneity by sampling the betweenrial standard deviation, τ (not the betweentrial variance τ ^{2}), from a uniform distribution, τ ~U(10 ^{10}, SQRT(0.60)). We simulated the underlying true trial intervention effects, as log odds ratio ln(OR _{ i })~N(OR, τ ^{2}), where OR is the true intervention effect expressed as an odds ratio. We drew the observed number of events in the intervention group from a binomial distribution e _{ iE } ~bin(n _{ i },PE _{ i }), where PE _{ i }= PC _{ i } exp(ln(OR _{ i }))/(1  PC _{ i }+ PC _{ i } exp(ln(OR _{ i })))
For all metaanalysis scenarios we simulated 10,000 metaanalyses and for each of these we calculated the and the . For each scenario we plotted D ^{2} against I ^{2} and incorporated the line of unity in the scatterplot.
2.5 Selection of metaanalyses examples
We selected traditional randomeffects metaanalyses to cover a range of inconsistency I ^{2} from 0% to 100% and to come from a wide range of medical research fields.
Results
3.1 The relationship between diversity, D^{2}, and heterogeneity, I^{2}
We want to show that D ^{2} ≥ I ^{2} for all metaanalyses. This is true if and only if:
According to a special case of the Chebyshev's inequality [13] we arrange the weights so w _{1} ≥ w _{2} ≥ ...... w _{ k }., for any k ≥ 0 we then get that:
and hence:
and therefore
and subsequently:
We remember that Takouche et al. [14] proposed an estimate of a 'typical' sampling error , which yields the following relationship between and :
So it follows from (3.5) that for all metaanalyses. Furthermore if we apply Chebyshev's inequality [13] arranging the weights and at the same time w _{1} ≥ w _{2} ≥ ...... w _{ k }then:
and as the randomeffects weights are and the fixedeffect weights are we get:
and hence:
which leads to:
and subsequently:
and since:
it follows that:
and therefore:
Remembering the definitions of and lead to:
and it appears from (3.7) that for all metaanalyses. As we have already shown in (3.5) that it becomes clear that in all metaanalyses. As and with it follows that:
and, finally, D ^{2} ≥ T ^{2} ≥ I ^{2} in all metaanalyses.
3.2 Some useful properties of D^{2}
Higgins and Thompson [10] specified three criteria that should be met by any quantification of variability between trials included in a metaanalysis: 1) the quantity should be a monotonically increasing function of the betweenstudy variance, τ ^{2}; 2) the quantity should be scale invariant; and 3) the quantity should be independent of the number of included trials. It is easily verified that the D ^{2} fulfils the first two of these criteria. The third criterion may not be fulfilled, even by I ^{2} (simulations by K Thorlund, personal communication). However, D ^{2} becomes independent of the numbers of trials included in the metaanalysis, to the same degree as , because D ^{2} is a transformation of fulfilling the criterion according to Higgins and Thompson [10]. Furthermore, it is easy to show that:
demonstrating that the percentage of increase in variance when the model of metaanalysis is changed from a fixedeffect model into a randomeffects model can, of course, also be expressed in terms of diversity.
It is equally clear that D ^{2} is always ≥ 0 as well as being < 1. D ^{2} is a fraction between 0 and 100% because:
as (1 + w _{i}·τ ^{2}) ≥ 1 for all i and for all estimators of τ ^{2} including the DerSimonianLaird estimator [15] with being at least greater than or equal to 0.
Furthermore, D ^{2} = I ^{2} when and only when all the weights w _{i} in the fixedeffect model are equal. D ^{2}is approximately equal to I ^{2} if:
Furthermore, D ^{2} = 0 when and only when I ^{2} = 0 because I ^{2} = 0 when and only when τ ^{2} = 0 the latter making = 1 and hence .
3.3 Simulations of metaanalyses
We performed 8 simulation scenarios showing that D ^{2} always exceeds I ^{2} despite any assumptions. Metaanalyses with all weights being equal corresponding to D ^{2} = I ^{2} were rare. The pattern of data showed a greater degree of scatter in the scenarios where k = 6. The results of the simulations of 10 000 metaanalyses according to the combinations of OR = 0.70, OR = 1.00 and PC = 30% with 6 and 20 trials, respectively, are presented in figure 1 and figure 2. As seen D ^{2} exceeds I ^{2} for all the simulated metaanalyses independent of the chosen OR and number of trials in the metaanalyses.
3.4 Examples
We used the expression of D ^{2} to calculate this quantity in seven traditional randomeffects metaanalyses [16–22] listed in Table 1. These metaanalyses cover a range of inconsistency, I ^{2}, from 0% to 74.2% and come from different medical research fields: intensive care [16], assisted reproductive technology [17], perioperative medicine [19, 22], neonatology [18, 20], and cardiology [21]. The results of the calculations of I ^{2},D ^{2}, inconsistencyadjusted information size HIS (), and diversityadjusted information size DIS () from these metaanalyses are shown in Table 2. The range of the calculated unadjusted SS range from 440 to 31,094 participants.
Figure 3 shows the relationship between D ^{2}, I ^{2}, and unity. All the metaanalyses examples are shown as open circles above the line of unity as D ^{2} ≥ I ^{2}. The difference (D ^{2}  I ^{2}) increases with heterogeneity until a certain point, after which the difference again regresses to 0.
Discussion
Using a mathematical derivation, metaanalyses simulations, and examples of metaanalyses we derive a concept of diversity, D ^{2}. D ^{2} may be used for adjustment of the required information size in any randomeffects model metaanalysis once the between trial variance is estimated. Focusing on the required information size estimation in a randomeffects metaanalysis, D ^{2} seems less biased compared to I ^{2}. The D ^{2} is directly constructed to fulfil the requirements of the information size calculation and is subsequently independent of any 'typical' a priori sampling error estimate, whereas the I ^{2} is influenced by an a priori 'typical' sampling error estimate. We therefore find that it is possible and appropriate taking D ^{2} into consideration to calculate the required IS in metaanalyses as DIS.
DIS has several advantages. It measures the required IS needed to preserve the anticipated risk of type I and type II errors in a randomeffects model metaanalysis. DIS considers total variance change when the model shifts from a fixedeffect into a randomeffects model. DIS is a model dependent and derived estimate of the required IS. The adjustment is dependent only on the anticipated intervention effect and on the model used to incorporate the betweentrial variance estimate . D ^{2} applies to randomeffects models other than that proposed by DerSimonianLaird [16] as long as the betweentrial estimator, , is specified. The adjustment of IS does not depend on the level of type I and II errors, as (Z _{1α/2 }+ Z _{1β })^{2} is levelled out during the derivation of the adjustment factor A _{ RF }(see equation 2.1, 2.2, and 2.5). The relationship D ^{2} ≥ I ^{2} in all the simulations and in all the examples (shown as points above the line of unity in figure 1, 2, and 3) are in accordance with the properties of D ^{2} compared to I ^{2} derived in section 3.1.
There are limitations of DIS. Like HIS the use of DIS cannot compensate for systematic bias such as selection bias, allocation bias, reporting bias, collateral intervention bias, and time lag bias [5, 23–28]. Furthermore, DIS is always greater than or equal to HIS, which may emphasise that caution is needed when interpreting metaanalysis before the required DIS has been reached [2–8].
The calculation of HIS and DIS may seem to contrast the SS calculation in a single trial where no adjustment for heterogeneity or diversity is performed. However, Fedorov and Jones [29] advocated the necessity of adjusting SS for heterogeneity arising from different accrual numbers among centres in a multicentre trial in order to avoid the trial being underpowered. If such an adjustment seems fair for a single trial, it also appears appropriate for a metaanalysis of several trials. As an example, we calculated the DIS to 14,164 participants for a metaanalysis of the effect on mortality of perioperative betablockade in patients for noncardiac surgery (Table 2). This may explain why a recent metaanalysis of seven randomised trials with lowrisk of bias including 11,862 participants indicates, but still does not convincingly show, firm evidence for harm [30]. The actual accrual of 11,862 participants is beyond the HIS of 9,726 participants, but below the DIS of 14,164 participants, and the metaanalysis [30] may still be inconclusive. This suggest that HIS is not a sufficiently adjusted metaanalytic information size. Furthermore, the example demonstrates the important question of the stability of I ^{2} and D ^{2} beyond a certain number of trials in a metaanalysis as I ^{2} was 13.4% in the metaanalysis after 2,211 participants [19] and has now doubled to I ^{2} = 27.0% after 11,862 accrued participants in the metaanalysis of seven trials with lowrisk of bias [30]. The assumption of I ^{2} and D ^{2} becoming stable after five trials is probably wrong and illustrates the moving target concept, which we have to face doing cumulative metaanalysis as evidence accumulates. Although a moving target may cause conceptual problems, a moving target may be better than no target at all.
The assumption that the IS required for a reliable and conclusive fixedeffect metaanalysis should be as large as the SS of a single wellpowered randomised clinical trial to detect or reject an anticipated intervention effect [2–4] may not be necessary in some instances. The statistical information (SINF) required in a metaanalysis could ultimately be expressed as [31], with δ being the effect size. As SINF is the reciprocal of the variance in the metaanalysis, say , it follows that in metaanalyses with , the amount of information may eventually suffice to detect, or reject, an effect size of δ, without yet having reached HIS or DIS. This criterion, however, is not a simple one and may only be fulfilled occasionally. Furthermore, it seems impossible to forecast or even to get an idea of the magnitude of in the beginning of a series of trials as well as along the course of trials being performed.
D ^{2} offers a number of useful properties compared to I ^{2}. In contrast to I ^{2}, D ^{2} reflects the relative variance expansion due to the between trial variance estimate without assuming an estimate of a 'typical' sampling error σ ^{2}. D ^{2} is reduced when the estimate is reduced, even for the same set of trials. In case diversity is larger than inconsistency this may be an indication that total variability among trials in the metaanalysis is even greater than suggested by I ^{2}. I ^{2} is intrinsically influenced by a potentially overestimated sampling error (), thereby underestimating and inherently placing less weight on large trials with many events. On the other hand a 'typical' sampling error originating from the required information size, , could be deduced from the D ^{2}. We would, however, advise great cautiousness in such an attempt. The difference (D ^{2}  I ^{2}) reflects the difference of the momentbased and the information sizebased 'typical' sampling error estimate. The calculation of diversity and (D ^{2}  I ^{2}) may serve as supplementary tools to the assessment of variability in a metaanalysis. D ^{2} is a transformation of the variance ratio of the variances from the randomeffects model and the fixedeffect model. This variance ratio was a candidate for the quantification of heterogeneity [10].
D ^{2} may vary within the same set of trials when different between trial variance estimators are used in the corresponding randomeffects model. On the contrary, I ^{2} is intimately linked to the specific between trial variance estimator in the DerSimonianLaird randomeffects model as I ^{2} by definition is [10] and Q is used to estimate a momentbased between trial variance [15]. The interpretation of heterogeneity is obviously dependent on the variance estimator as well. An estimate of τ ^{2} is a prerequisite for any randomeffects model and the actual estimated value, together with the way is incorporated into the model, actually constitutes the model [32]. Therefore, a quantification of the betweentrial variability rather than sampling error which is independent of the specific randomeffects model is impossible, as it is constituted by the between trial variance estimator [32]. D ^{2} adapt automatically to different between trial variance estimators [32] while I ^{2} is linked to the estimator from the DerSimonianLaird randomeffects model.
D ^{2} may have some limitations too. The derivation of D ^{2} depends on the assumption that the point estimate of the intervention effect in the fixedeffect model and the point estimate of the intervention effect in the randomeffects model are approximately equal. Metaanalyses with considerable difference of the point estimate in the fixedeffect model and the point estimate in the randomeffects model represent specific problems. Probably more information is needed when μ _{ F }>> μ _{ R }since the formula yields higher values for N _{ R }under the assumption of a constant variance ratio. On the other hand less information may be needed when μ _{ F }<<μ _{ R }since the formula then yields lower values for N _{ R }under the assumption of a constant variance ratio. However, examples with considerable differences of the point estimates in a fixed and randomeffects model presumably represent metaanalyses of interventions with considerable between trial variance due to small trial bias. The metaanalysis of the effect of magnesium in patients with myocardial infarction is such an example [21] where one large trial totally dominate the result in the fixedeffect model but are unduly downweighted in the randomeffects model. Care should be taken to interpret the randomeffects model despite any calculated information size in such a situation. Further, to foresee a priori the size of the difference between μ _{ F }and μ _{ R }seems impossible and the calculation may then degenerate exclusively to a post hoc analysis.
Second, D ^{2}, though potentially unbiased with respect to information size calculations, could come with a greater variance than I ^{2} when both are calculated in the same set of metaanalyses. This latter situation presents a potentially unfavourable 'biasvariancetrade off' but an estimate of its magnitude will have to await simulation studies addressing the issue.
It may seem an advantage that I ^{2} is always reported in metaanalysis and therefore readily available to adjust the expected information size. On the other hand is also calculable for metaanalysis of ratio measures (e.g, RR or OR), width_{F} and width_{R} refers to the widths of the confidence intervals for the logarithmic transformed measures in the fixedeffect and the randomeffects models, respectively.
Last but not least the decision to pool intervention effect estimates in metaanalysis should be the clinical relevance of any inconsistency or diversity present. The between trial variance,τ ^{2}, rather than I ^{2} or D ^{2}, may be the appropriate measure for this purpose [33–35].
The estimation of a required IS for a metaanalysis to detect or reject an anticipated intervention effect on a binary outcome measure should be considered based on reasonable assumptions. Accordingly, it may not be wise to assume absence of heterogeneity in a metaanalysis unless the intervention effect is anticipated to be zero [36, 37]. On the contrary it may be wise to anticipate moderate to substantial heterogeneity (e.g., more than 50%) in an a priori adjustment of the required IS [37]. The concept of diversity points to the fact that an adjustment based on the experience with inconsistency would result in underestimated heterogeneity and hence an underestimated required IS [37]. Alternatively for a future updated metaanalysis to become conclusive we may apply the actual estimated heterogeneity of the available trials in a metaanalysis as the best we have for the adjustment of the required IS. D ^{2} seems more capable than I ^{2} in obtaining such an adequate adjustment.
Conclusion
A quantity to characterise the proportion of between trial variation in any metaanalysis relative to the total model variance of the included trials is needed. Diversity, D ^{2}, may be such a quantity. D ^{2} describes the relative model variance reduction changing from a randomeffects model into a fixedeffect model. Diversity may be described as the proportion of the total variance in a randomeffects model contributed by the between trial variation despite the chosen between trial variance estimator. Furthermore, D ^{2} can adequately adjust the required information size in any randomeffects metaanalysis irrespective the metaanalytic model.
Conflicts of interests
The authors declare that they have no competing interests.
Authors information
JW is an anaesthesiologist and a trialist working with metaanalysis and trial sequential analysis at the Copenhagen Trial Unit having special interests in perioperative medicine.
KT is a biostatistician working with metaanalysis and trial sequential analysis at the Copenhagen Trial Unit.
JB is an intern working in paediatrics with metaanalysis and trial sequential analysis.
CG is head of the Copenhagen Trial Unit, EditorInChief of the Cochrane HepatoBiliary Group, a trialist, and an associate professor at Copenhagen University.
Abbreviations
 α :

Risk of type 1 error
 β :

Risk of type 2 error
 A _{ RF } :

Adjustment factor of information size changing from a fixedeffect to a randomeffects model
 ∀:

For any...
 Q :

Cochran's Q
 D ^{2} :

Diversity
 DIS :

Diversity adjusted information size ()
 HIS :

Heterogeneity adjusted information size ()
 I ^{2} :

Inconsistency factor
 K :

Number of trials in a metaanalysis
 N _{R} :

Required number of participants in a randomeffects metaanalysis
 N _{F} :

Required number of participants in a fixedeffect metaanalysis
 IS :

Required number of participants in a metaanalysis
 μ _{F} :

Estimate of the intervention effect in a fixedeffect metaanalysis
 μ _{R} :

Estimate of the intervention effect in a randomeffects metaanalysis
 OR :

Odds ratio
 PC :

Control event rate
 RRR :

Relative risk reduction
 SS :

Sample size in a single randomised clinical trial
 :

Estimate of a typical sampling error considering diversity
 :

Estimate of a typical momentbased sampling error
 :

Mean of estimates of sampling errors in a metaanalysis
 τ ^{2} :

Estimator of the variance of between trial intervention effect estimates
 :

Estimate of the variance of between trial intervention effect estimates
 :

DerSimonianLaird estimate of the variance of between trial intervention effect estimates
 V _{ F } :

The variance in a fixedeffect metaanalysis
 V _{ R } :

The variance in a randomeffects metaanalysis
 Z _{1α/2} Fractile for 1α :

/2
 Z _{1β } Fractile for 1β :

.
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Acknowledgements
We are grateful to Jørgen Hilden, M.D., associate professor emeritus at the Department of Biostatistics, Copenhagen University, for having critically reviewed a former version of our manuscript. We thank the peer reviewers Rebecca Turner, MSc in statistics and Gerta Rücker, MSc in statistics for helpful suggestions for improvements of the manuscript.
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JW and KT conceived the idea of an information size adjustment factor and JW devised the concept of diversity, made the mathematical derivations and calculated the examples. KT sat up the simulation program and KT and JW performed the simulations. JW, KT, and CG drafted the first manuscript. KT, JB and CG suggested revisions implemented by JW.
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Wetterslev, J., Thorlund, K., Brok, J. et al. Estimating required information size by quantifying diversity in randomeffects model metaanalyses. BMC Med Res Methodol 9, 86 (2009) doi:10.1186/14712288986
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Keywords
 Sampling Error
 Information Size
 Trial Variance
 Trial Sequential Analysis
 Total Model Variance