Following recommendations by Sterne et al. (2011), by default, a
test for funnel plot asymmetry is only conducted if the number of
studies is ten or larger (argument k.min=10
). This behaviour
can be changed by setting a smaller value for argument
k.min
. Note, the minimum number of studies is three.
If argument method.bias
is "rank"
, the test statistic
is based on the rank correlation between standardised treatment
estimates and variance estimates of estimated treatment effects;
Kendall's tau is used as correlation measure (Begg & Mazumdar,
1994). The test statistic follows a standard normal distribution. By
default (if correct
is FALSE), no continuity correction is
utilised (Kendall & Gibbons, 1990).
If argument method.bias
is "linreg"
, the test
statistic is based on a weighted linear regression of the treatment
effect on its standard error (Egger et al., 1997). The test
statistic follows a t distribution with number of studies - 2
degrees of freedom.
If argument method.bias
is "mm"
, the test statistic is
based on a weighted linear regression of the treatment effect on its
standard error using the method of moments estimator for the
additive between-study variance component (method 3a in Thompson,
Sharp, 1999). The test statistic follows a t distribution with
number of studies - 2
degrees of freedom.
If argument method.bias
is "peters"
, the test
statistic is based on a weighted linear regression of the treatment
effect on the inverse of the total sample size using the variance of
the average event rate as weights (Peters et al., 2006). The test
statistic follows a t distribution with number of studies - 2
degrees of freedom. This test is available for meta-analyses
comparing two binary outcomes or combining single proportions, i.e.
generated with functions metabin
and metaprop
.
The following tests for funnel plot asymmetry are only available for
meta-analyses comparing two binary outcomes, i.e. meta-analyses
generated with the metabin
function.
If argument method.bias
is "count"
, the test statistic
is based on the rank correlation between a standardised cell
frequency and the inverse of the variance of the cell frequency;
Kendall's tau is used as correlation measure (Schwarzer et al.,
2007). The test statistic follows a standard normal distribution. By
default (if correct
is FALSE), no continuity correction is
utilised (Kendall & Gibbons, 1990).
If argument method.bias
is "score"
, the test statistic
is based on a weighted linear regression utilising efficient score
and score variance (Harbord et al., 2006). The test statistic
follows a t distribution with number of studies - 2
degrees
of freedom.
In order to calculate an arcsine test for funnel plot asymmetry
(R<U+00FC>cker et al., 2008), one has to use the metabin
function
with argument sm="ASD"
as input to the metabias
command. The three arcsine tests described in R<U+00FC>cker et al. (2008)
can be calculated by setting method.bias
to "rank"
,
"linreg"
and "mm"
, respectively.
If argument method.bias
is missing, the Harbord test
(method.bias="score"
) is used for the odds ratio as effect
measure and the Egger test (method.bias="linreg"
) for other
effect measures (Sterne et al., 2011).
No test for funnel plot asymmetry is conducted in meta-analyses with
subgroups.