[R-meta] About whether to delete the outliers from the dataset

[Reza Norouzian]

Dear Nick,

It may be useful to add some additional context to your question for better
assistance.

For example, do you have a multilevel data structure where each study could
have multiple rows or instead you have allowed only one row for each study
in your dataset?

Also, can you possibly describe your method of outlier detection? For
instance, if you are using the metafor package, have you looked at the
combination of cooks.distance(), hatvalues(), and rstudent() for those
large effects in your meta-regression model?

Additionally, I wonder what happens to your pooled effect's standard error
(or the width of the pooled effect's confidence interval [CI]) with versus
without those large effects? For example, does the width of the CI
substantially (ex. by ~30%) decrease after removing those large effects,
increase, or remain largely unchanged?

Finally, depending on how much this matters to you in terms of your study
objectives, does retaining versus removing those large effects in your
meta-regression model change the statistical significance of your pooled
effect at all (i.e., sig. to not sig., or vice versa)?

Reza


On Sun, Dec 10, 2023 at 12:33 AM Nick Chen via R-sig-meta-analysis <
r-sig-meta-analysis using r-project.org> wrote:

> I have a question concerning whether to delete some of the data or not. I
> have a dataset of 56 studies with a pooled effect size of g=1.25. Yet,
> there are 4 data that reported an incredibly high effect size (8.15, 6.63,
> 4.14, 4.10 respectively). Statistically, they should be considered as
> outliers and be removed from the dataset. But since these data went through
> the inclusion and exclusion criteria, they should be staying in the dataset
> since they met all the requirements of my selection. So if we excluded the
> 4 data, wouldn't that be miss-reporting some data in the dataset? What
> should I do? Should I excluded the 4 seemingly influential cases or keep
> them for a complete list of research?
> *Name*: Nick Chen (Ping-Cheng, Chen)
> *School*:National Taiwan Normal University (NTNU) English Department
> (Master)
> *Email*: wow99308008 using gmail.com
> *Phone number*: +886 909 663 963
>
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