tips:rma.uni_vs_rma.mv

The main workhorse of the metafor package is the `rma.uni()`

function (same as `rma()`

), which fits meta-analytic fixed-, random-, and mixed-effects models using what is often referred to as the "inverse-variance method" (for random/mixed-effects models, this is also called the "normal-normal model", as it assumes normally distributed sampling distributions with known sampling variances and normally distributed random effects for the total/residual amount of heterogeneity).

For multilevel, multivariate, and network meta-analyses, the package also provides the `rma.mv()`

function, which allows for correlated sampling errors and/or true effects. Of course, one can also use the `rma.mv()`

function to fit the same models as the `rma.uni()`

function, since those models are really just special cases of the more general models handled by the `rma.mv()`

function. This is illustrated below.

**Note:** Alternative modeling approaches for particular types of data (e.g., 2×2 table data, two-group person-time data, proportions, incidence rates) are also available, including the Mantel-Haenszel method (`rma.mh()`

function), Peto's (one-step) method (`rma.peto()`

function), and a variety of suitable mixed-effects (conditional) logistic and Poisson regression models (`rma.glmm()`

function).

To illustrate the use of the `rma.uni()`

and `rma.mv()`

functions for fitting fixed-effects models, let's use the BCG vaccine data. We can compute the log risk ratios and corresponding sampling variances with:

library(metafor) dat <- escalc(measure="RR", ai=tpos, bi=tneg, ci=cpos, di=cneg, data=dat.bcg)

trial author year tpos tneg cpos cneg ablat alloc yi vi 1 1 Aronson 1948 4 119 11 128 44 random -0.8893 0.3256 2 2 Ferguson & Simes 1949 6 300 29 274 55 random -1.5854 0.1946 3 3 Rosenthal et al 1960 3 228 11 209 42 random -1.3481 0.4154 4 4 Hart & Sutherland 1977 62 13536 248 12619 52 random -1.4416 0.0200 5 5 Frimodt-Moller et al 1973 33 5036 47 5761 13 alternate -0.2175 0.0512 6 6 Stein & Aronson 1953 180 1361 372 1079 44 alternate -0.7861 0.0069 7 7 Vandiviere et al 1973 8 2537 10 619 19 random -1.6209 0.2230 8 8 TPT Madras 1980 505 87886 499 87892 13 random 0.0120 0.0040 9 9 Coetzee & Berjak 1968 29 7470 45 7232 27 random -0.4694 0.0564 10 10 Rosenthal et al 1961 17 1699 65 1600 42 systematic -1.3713 0.0730 11 11 Comstock et al 1974 186 50448 141 27197 18 systematic -0.3394 0.0124 12 12 Comstock & Webster 1969 5 2493 3 2338 33 systematic 0.4459 0.5325 13 13 Comstock et al 1976 27 16886 29 17825 33 systematic -0.0173 0.0714

A fixed-effects model can be fitted to these data using the `rma.uni()`

function with:

rma(yi, vi, method="FE", data=dat)

Fixed-Effects Model (k = 13) Test for Heterogeneity: Q(df = 12) = 152.2330, p-val < .0001 Model Results: estimate se zval pval ci.lb ci.ub -0.4303 0.0405 -10.6247 <.0001 -0.5097 -0.3509 *** --- Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

Again, using `rma()`

is the same as invoking the `rma.uni()`

function, but the former is quicker to type.

The same model can be fitted to these data using the `rma.mv()`

function with:

rma.mv(yi, vi, data=dat)

Multivariate Meta-Analysis Model (k = 13; method: REML) Variance Components: none Test for Heterogeneity: Q(df = 12) = 152.2330, p-val < .0001 Model Results: estimate se zval pval ci.lb ci.ub -0.4303 0.0405 -10.6247 <.0001 -0.5097 -0.3509 *** --- Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

The results are exactly the same.

In both functions, predictors/covariates/moderators can be included in the model via the `mods`

argument. For example:

rma(yi, vi, mods = ~ ablat + year, method="FE", data=dat) rma.mv(yi, vi, mods = ~ ablat + year, data=dat)

Again, the results are the same (output not shown).

A random-effects model can be fitted to these data using the `rma.uni()`

function with:

rma(yi, vi, data=dat)

Random-Effects Model (k = 13; tau^2 estimator: REML) tau^2 (estimated amount of total heterogeneity): 0.3132 (SE = 0.1664) tau (square root of estimated tau^2 value): 0.5597 I^2 (total heterogeneity / total variability): 92.22% H^2 (total variability / sampling variability): 12.86 Test for Heterogeneity: Q(df = 12) = 152.2330, p-val < .0001 Model Results: estimate se zval pval ci.lb ci.ub -0.7145 0.1798 -3.9744 <.0001 -1.0669 -0.3622 *** --- Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

The default for the `method`

argument is to use REML estimation for the amount of heterogeneity and a random-effects model is then automatically fitted.

**When using the rma.mv() function, random effects must be explicitly added to the model via the random argument.** For a standard random-effects model, we need to add random effects for the trials, which can be done with:

rma.mv(yi, vi, random = ~ 1 | trial, data=dat)

Multivariate Meta-Analysis Model (k = 13; method: REML) Variance Components: estim sqrt nlvls fixed factor sigma^2 0.3132 0.5597 13 no trial Test for Heterogeneity: Q(df = 12) = 152.2330, p-val < .0001 Model Results: estimate se zval pval ci.lb ci.ub -0.7145 0.1798 -3.9745 <.0001 -1.0669 -0.3622 *** --- Signif. codes: 0 ‘***’ 0.001 ‘**’ 0.01 ‘*’ 0.05 ‘.’ 0.1 ‘ ’ 1

These are the same results as obtained earlier.^{1)}

In essence, the meta-analytic random-effects model can be conceptualized as a multilevel model with the true effects at level 2 and the observed effects at level 1. Using typical multilevel model terminology, the `random = ~ 1 | trial`

argument adds random intercepts at level 2 to the model. Note that the variance of the true effects (commonly denoted as $\tau^2$ in the meta-analytic literature) is denoted as $\sigma^2$ in the output above (i.e., the value of $\sigma^2$ given in the output is the estimated variance of the random intercepts).

Again, predictors/covariates/moderators can be included in the model via the `mods`

argument. For example:

rma(yi, vi, mods = ~ ablat + year, data=dat) rma.mv(yi, vi, mods = ~ ablat + year, random = ~ 1 | trial, data=dat)

The results are again the same (output not shown).

Due to slightly different optimization routines used in the

`rma.uni()`

and `rma.mv()`

functions, minor numerical differences are possible.tips/rma.uni_vs_rma.mv.txt · Last modified: 2018/05/31 06:48 by Wolfgang Viechtbauer

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