[R-meta] How does rma handle effect size of zero

[Viechtbauer, Wolfgang (SP)]

Hi Divya,

There is no special handling of zero effect sizes.

The p-value you are referring to corresponds to the test of H0: mu = 0. If the pooled estimate is exactly 0, then this obviously results in a p-value of 1. The same thing will happen also here:

rma(c(-.2, -.2, .2, .2), rep(.01, 4))

Best,
Wolfgang

>-----Original Message-----
>From: R-sig-meta-analysis [mailto:r-sig-meta-analysis-bounces using r-project.org]
>On Behalf Of Divya Ravichandar
>Sent: Monday, 08 June, 2020 22:01
>To: r-sig-meta-analysis using r-project.org
>Subject: Re: [R-meta] How does rma handle effect size of zero
>
>Hi all
>
>Apologies for a second email but I wanted to clarify the code provided in
>the example above. Corrected code below
>
>library(metafor)
>library(magrittr)
>case1 <- data.frame(Study= c("a","b","c","d"),ES=rep(.1,4),SE=rep(1e-5,4))
>%>% rma(ES, SE^2, data=.) #non zero case
>case2 <- data.frame(Study= c("a","b","c","d"),ES=rep(0,4),SE=rep(1e-5,4))
>%>% rma(ES, SE^2, data=.) #zero case
>
>On Mon, Jun 8, 2020 at 12:59 PM Divya Ravichandar <divya using secondgenome.com>
>wrote:
>
>> Hi all
>>
>> I would like to get some understanding around how rma handles effect sizes
>> of 0. A test example is outlined below
>>
>> library(metafor)
>> library(magrittr)
>>
>> case1 <- data.frame(Study= c("a","b","c","d"),ES=rep(.1,4),SE=rep(1e-5,4))
>> %>% rma(ES, SE^2, data=.) #non zero case
>> meta_case1 <- rma(ES, SE^2,  data=case1)
>> case2 <- data.frame(Study= c("a","b","c","d"),ES=rep(0,4),SE=rep(1e-5,4))
>> %>% rma(ES, SE^2, data=.) #zero case
>>
>> While case 1 results in a significant p value  (<.0001), case 2 results in
>> a non-significant p value (1). Does a zero effect size violate any
>> assumptions and if not I wonder why a consistent estimate of 0 across
>> datasets results in a non-significant result?
>>
>> Thanks
>>
>> --
>> *Divya Ravichandar*
>> Scientist
>> Second Genome