I have no idea how many people take the effort to reproduce
a meta-analysis in their spare time. What I do know, based on my personal
experiences of the last week, is that A) it’s too much work to reproduce a meta-analysis,
primarily due to low reporting standards, and B) we need to raise the bar
when doing meta-analyses. At the end of this post, I’ll explain how to do a
meta-analysis in R in five seconds (assuming you have effect sizes and sample
sizes for each individual study) to convince you that you can produce (or
reproduce) meta-analyses yourself.

Any single study is nothing more than a data-point in a
future meta-analysis. In the last years researchers have shared a lot of
thoughts and opinions about reporting standards for individual studies, ranging
from disclosure statements, additional reporting of alternative statistical results
whenever a researcher had the flexibility to choose from multiple analyses, to
sharing all the raw data and analysis files. When it comes to meta-analyses, reporting
standards are even more important.

Recently I tried to reproduce a meta-analysis (by Sheinfeld
Gorin, Krebs, Badr, Janke, Jim, Spring, Mohr, Berendsen, & Jacobsen, 2012,
with the titel “Meta-Analysis of Psychosocial Interventions to Reduce Pain in
Patients With Cancer” in the Journal of Clinical Oncology, which has an IF of
18, and the article is cited 38 times) for a talk about statistics and
reproducibility at the International Conference of Behavioral Medicine.

**Of the 38 effect sizes included in the meta-analysis I could reproduce 27 effect sizes (71%). Overall, I agreed with the way the original effect size was calculated for 18 articles (47%).**I think both these numbers are too low. It could be my lack of ability in calculating effect sizes (let's call it a theoretical possibility) and I could be wrong in all cases in which I disagreed with which effect size to use (I offered the authors of the meta-analysis the opportunity to comment on this blog post, which they declined). But we need to make sure meta-analyses are 100% reproducible, if we want to be able to discuss and resolve such disagreements.
For three papers, statistics were not reported in enough detail
for me to calculate effect sizes. The researchers who performed the meta-analysis
might have contacted authors for the raw data in these cases. If so, it is
important that authors of a meta-analysis share the summary statistics their effect
size estimate is based on. Without additional information, those effect sizes
are not reproducible by reviewers or readers. After my talk, an audience member noted that sharing data you have gotten from someone would require their permission - so if you ask for additional data when doing a meta-analysis, also ask to be able to share the summary statistics you will use in the meta-analysis to improve reproducibility.

For 9 studies, the effect sizes I calculated differed
substantially from those by the authors of the meta-analysis (so much that it's not just due to rounding differences). It
is difficult to resolve these inconsistencies, because I do not know how the
authors calculated the effect size in these studies. Meta-analyses should give
information about the data effect sizes are based on. A direct quote from the
article that contains the relevant statistical test, or pointing to a row and
column in a Table that contains the means and standard deviations would have
been enough to allow me to compare calculations.

We might still have disagreed about which effect size should
be included, as was the case for 10 articles where I could reproduce the effect
size the authors included, but where I would use a different effect size
estimate. The most noteworthy disagreement probably was a set of three articles
the authors included, namely:

*de Wit R, van Dam F, Zandbelt L, et al: A pain education program for chronic cancer pain patients: Follow-up results from a randomized controlled trial. Pain 73:55-69, 1997*

*de Wit R, van Dam F: From hospital to home care: A randomized controlled trial of a pain education programme for cancer patients with chronic pain. J Adv Nurs 36:742-754, 2001a*

*de Wit R, van Dam F, Loonstra S, et al: Improving the quality of pain treatment by a tailored pain education programme for cancer patients in chronic pain. Eur J Pain 5:241-256, 2001b*

The authors of the meta-analyses calculated three effect
sizes for these studies: 0.21, 0.14, and -0.19. I had a research assistant
prepare a document with as much statistical information about the articles as
possible, and she noticed that in her calculations, the effect sizes of De Wit
et al (2001b) and De Wit et al (1997) were identical. I checked De Wit 2001a
(referred to as De Wit 2002 in the forest plot in the meta-analysis) and noticed
that all three studies reported the data of 313 participants. It’s the same
data, written up three times. It’s easy to miss, because the data is presented
in slightly different ways, and there are no references to earlier articles in
the later articles (in the 2001b article, the two earlier articles are in the
reference list, but not mentioned in the main text). I contacted the
corresponding second author for clarifications, but received no reply (I would still be happy to add any comments I receive). In any
case, since this is the same data, and since the effect sizes are not independent, it
should only be included in the meta-analysis once.

A second disagreement comes from Table 2 in Anderson,
Mendoza, Payne, Valero, Palos, Nazario, Richman, Hurley, Gning, Lynch, Kalish,
and Cleeland (2006), Pain Education for Underserved Minority Cancer Patients: A
Randomized Controlled Trial, also published in the Journal of Clinical
Oncology), reproduced below:

See if you can find the seven similar means and standard deviations
– a clear copy-paste error. Regrettably, this makes it tricky to calculate the
difference on the Pain Control Scale, because they might not be correct. I
contacted the corresponding first author for clarifications, but have received
no reply (but the production office of the journal is looking in to it).

There are some effect size calculations where I strongly
suspect errors were made, for example because adjusted means from an ANCOVA
seem to be used instead of unadjusted means, or the effect size seems to be
based on part of the data (only post-scores instead of the differences on Time1
and Time2 change scores, or the effect size in the intervention condition
instead of the difference between the intervention and control condition). To
know this for sure, the authors should have shared the statistics their effect
size calculations were based on. I could be wrong, but disagreements can only be resolved if the data the effect sizes are calculated on is clearly communicated together with the meta-analysis.

**The most important take home message at this point should be that A) there are enough things that researchers can disagree about if you take a close look at published meta-analyses, and B) the only way to resolve these disagreements is by full disclosure about how the meta-analysis was performed.**All meta-analyses should include a meta-analysis disclosure table with the publication which provides a detailed description of the effect sizes that were used, including copy-pasted sentences from the original article or references to rows and columns in Tables that contain the relevant data. In p-curve analyses (Simonsohn, Nelson, & Simmons, 2013) such disclosure tables are required, including alternative effects that could have been included and a description of the methods and design of the study. All meta-analyses should include a disclosure table with information on how effect sizes were calculated.### Inclusion Criteria: The Researcher Degrees of Freedom of the Meta-Analyst

The choice of which studies you do or do not include in a
meta-analysis is a necessarily subjective. It requires researchers to
determine what their inclusion criteria are, and to decide whether a study
meets their inclusion criteria or not. More importantly, if meta-analysts share
all the data their meta-analysis is based on, it’s easy for reviewers or
readers to repeat the analysis, based on their own inclusion criteria. In the
meta-analysis I checked, 3 types of interventions to reduce pain in cancer
patients were used. The first is pain management education, which involves
increasing knowledge about pain, how to treat pain, and when and how to contact
healthcare providers when in pain (for example to change their pain treatment).
The second is hypnosis, provided in individual sessions by a therapist, often
tailored to each patient, consisting of for example suggestions for pleasant
visual imagery and muscle relaxation. The third is relaxation and cognitive
coping skills, consisting of training and practice in relaxation exercises, attention
diversion, and positive affirmations.

When doing a random effects meta-analysis, effects under
investigation should be ‘different but similar’ and not ‘different and
unrelated’ (Higgins, Thompson, & Spiegelhalter, 2009).

**If there is heterogeneity in the effect size estimate, you should not just stop after reporting the overal effect size, but examine subsamples of studies.**I wanted to know whether the conclusion of a positive and effect size that was statistically different from zero over all studies would also hold for the subsamples (and whether the subsets would no longer show heterogeneity). It turns out that the evidence for pain management education is pretty convincing, while the effect size estimates for relaxation intervention was less convincing. The hypnosis intervention (sometimes consisting of only a 15 minute session) yielded effect sizes that were twice as large, but based on my calculations and after controlling for outliers, were not yet convincing. Thus, even though I disagreed on which effect sizes to include, based on the set of studies selected from the literature (which is in itself another interesting challenge for reproducibility!) the main difference in conclusions were based on which effects were 'different but similar'.**You can agree or disagree with my calculations. But what’s most important is that you should be able to perform your own meta-analysis on publically shared, open, and easily accessible data, to test your own ideas of which effects should and should not be included.**

### Performing a meta-analysis in R

I had no idea how easy doing a meta-analysis was in R (fun fact: when I was talking about this to someone, she pointed out the benefits of not sharing this too widely, to have an individual benefit of 'knowing how to do meta-analyses' - obviously, I think the collective benefit of everyone being able to do or check a meta-analysis is much greater). I did
one small-scale meta-analysis once (Lakens, 2012), mainly by hand, which was
effortful. Recently, I reviewed a paper by Carter
& McCullough (2014) where the authors were incredibly nice to share
their entire R script alongside their (very interesting) paper. I was amazed how easy it was to reproduce (or adapt) meta-analyses
this way. If this part is useful, credit goes to Carter and McCollough and
their R
script (their script contains many more cool analyses, such as tests of excessive significance, and PET-PEESE meta-regressions, which are so cool they deserve an individual blog post in the future).

All you need to have to do a meta-analysis is the effect
size for each study (for example Cohen’s d) and the sample size in each of the
two conditions Cohen’s d is based on. The first string es.d contains five
effect sizes from 5 studies. The n1 and n2 strings contain the sample sizes for
the control conditions (n1) and the experimental condition (n2). That’s all you
need to provide, and assuming you’ve calculated the effect sizes (not to brag,
but I found my own excel sheets to calculate
effect sizes that accompany my 2013
effect size paper very useful in this project) and coded the sample sizes,

**the rest of the meta-analysis takes 5 seconds**. You need to copy-paste the entire code below in R or RStudio (both are free) and first need to install the meta and metaphor packages. After that, you just insert your effect sizes and sample sizes, and run it. The code below is by Carter and McCullough, with some additions I made.
The output you get will contain the results of the
meta-analysis showing an overall effect size of d = 0.31, 95% CI [1.13; 0.50]:

95%-CI %W(fixed) %W(random)

1 0.38 [
0.0571; 0.7029] 33.62 33.62

2 0.41 [
0.0136; 0.8064] 22.32 22.32

3 -0.14 [-0.7387; 0.4587] 9.78
9.78

4 0.63
[-0.0223; 1.2823] 8.24 8.24

5 0.22
[-0.1470; 0.5870] 26.04 26.04

Number of studies combined: k=5

95%-CI z p.value

Fixed effect model 0.3148 [0.1275; 0.502] 3.2945 0.001

Random effects model 0.3148 [0.1275; 0.502]
3.2945 0.001

Quantifying heterogeneity:

tau^2 = 0; H = 1 [1; 2.12]; I^2 = 0% [0%;
77.8%]

Test of heterogeneity:

Q
d.f. p.value

3.75
4 0.4411

In addition, there’s a check for outliers and influential
cases, and a forest plot:

This is just the basics, but it hopefully has convinced you
that the calculations involved in doing a meta-analysis take no more than 5
seconds if you use the right software. Remember that you can easily share your R script, containing all your data (but don't forget a good disclosure table) and analyses when submitted your manuscript to a journal, or when it has been accepted for publication. Now go and reproduce.