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Evidence · Trials

How to read a forest plot, properly, from scratch

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Solved by a.almeida in post #10
Discontinuation handling is the methodological detail that most changes a result and gets the least attention. Read how missing data was imputed before reading the effect size. None of the above is medical advice and I am not qualified to give any.

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SC
s.cardosoTL224 Jun 2025#1

The question in the title: How to read a forest plot, properly, from scratch I will give what I have already checked below so nobody repeats it.

Reading STEP 4 (JAMA, 2021) for the population rather than the effect, which I have not done properly before.

The baseline table is more restrictive than the way the trial gets discussed here. Several of the questions in this category come from people who would not have been enrolled.

What is the honest way to describe what the trial says to somebody outside its population?

0 likes 13mo
PD
p.dialloTL21 Jul 2025#2

Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting.

23 likes 13mo
BJ
b.jankowiakTL3Regular5 Jul 2025#3
p.diallo, post #2: Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting. Go to post

That matches what I have seen, for whatever a single anecdote is worth.

10 likes in reply to #2 13mo
RM
r.mensahTL29 Jul 2025#4

Post #2 is right about the mechanism and I think understates the practical bit.

Absolute and relative effects answer different questions. Write down the event rate in each arm and the difference between them; everything quotable is derived from those two numbers.

Noting that the question and the thing people usually mean by it are different.

3 likes 13mo
YM
y.mensahTL3Wiki editor13 Jul 2025#5

Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit.

0 likes 13mo
CC
c.castellanosTL217 Jul 2025 · edited#6
s.cardoso, post #1: The question in the title: How to read a forest plot, properly, from scratch I will give what I have already checked below so nobody repeats it. Reading STEP 4 ( JAMA , 2021) for the population rather than the effect, which I have not done properly before. The baseline table is more restrictive than the way the trial gets discussed… Go to post

Picking up post #4: that is the part I would want checked first.

Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting.

1 like in reply to #1 12mo
NE
n.ekstromTL2Regular20 Jul 2025#7

On post #4 — agreed on the reasoning, with one qualification.

A treatment-policy estimand asks what happens to people assigned to a strategy, including those who abandon it. A hypothetical estimand asks what would have happened had everyone continued. Both are legitimate and they give different numbers.

I would treat that as a working assumption and revisit it.

0 likes 12mo
TK
t.karlsenTL223 Jul 2025#8

That is clearer than the version I had in my head. Thank you.

22 likes 12mo
CC
c.correiaTL226 Jul 2025#9
p.diallo, post #2: Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting. Go to post

Understood. Thank you for being specific about the limits of it.

10 likes in reply to #2 12mo
AA
a.almeidaTL2 Solution30 Jul 2025#10
c.correia, post #9: Understood. Thank you for being specific about the limits of it. Go to post

Discontinuation handling is the methodological detail that most changes a result and gets the least attention. Read how missing data was imputed before reading the effect size.

None of the above is medical advice and I am not qualified to give any.

10 likes in reply to #9 12mo
DO
dr_okonkwoTL4 Moderator2 Aug 2025#11
b.jankowiak, post #3: That matches what I have seen, for whatever a single anecdote is worth. Go to post

Absolute and relative effects answer different questions. Write down the event rate in each arm and the difference between them; everything quotable is derived from those two numbers.

4 likes in reply to #3 12mo
MP
m.perrinTL25 Aug 2025#12

Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit.

13 likes 12mo
DF
d.fontaineTL27 Aug 2025#13

Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting.

27 likes 12mo
IB
i.bakkenTL210 Aug 2025#14

Worth separating two things that post #10 runs together.

Composite endpoints should be read component by component. A composite driven entirely by its softest component is a different finding from one where the components move together.

This has been discussed before and I could not find the thread, so, again.

0 likes 12mo
VS
v.szaboTL3Analytical chemist13 Aug 2025 · edited#15
p.diallo, post #2: Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting. Go to post

Picking up post #12: that is the part I would want checked first.

A treatment-policy estimand asks what happens to people assigned to a strategy, including those who abandon it. A hypothetical estimand asks what would have happened had everyone continued. Both are legitimate and they give different numbers.

Scoping that to what I have actually seen rather than what I have read.

8 likes in reply to #2 11mo
VK
v.kirchnerTL216 Aug 2025#16

Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit.

Second-hand, so weight it accordingly.

19 likes 11mo
SK
s.karlsen_rphTL319 Aug 2025#17
OV
o.vukovicTL221 Aug 2025#18
v.kirchner, post #16: Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit. Second-hand, so weight it accordingly. Go to post

Good question, well framed, and I would like to see it answered properly.

0 likes in reply to #16 11mo
CL
customs_ledgerTL3Regular24 Aug 2025#19

This follows post #16 rather than contradicting it.

Absolute and relative effects answer different questions. Write down the event rate in each arm and the difference between them; everything quotable is derived from those two numbers.

Adding a source would improve this post and I do not have one to hand.

13 likes 11mo
PO
p.ostergaardTL227 Aug 2025#20
y.mensah, post #5: Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit. Go to post

Nothing to add, except that this is the answer I would give if asked.

26 likes in reply to #5 11mo
CH
c.haddadTL229 Aug 2025 · edited#21
r.mensah, post #4: Post #2 is right about the mechanism and I think understates the practical bit. Absolute and relative effects answer different questions. Write down the event rate in each arm and the difference between them; everything quotable is derived from those two numbers. Noting that the question and the thing people usually mean by it are… Go to post

Same experience here, different supplier, so it is at least not unique to one of them.

1 like in reply to #4 11mo
PN
plateau_notesTL2Regular1 Sep 2025#22
p.ostergaard, post #20: Nothing to add, except that this is the answer I would give if asked. Go to post

Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit.

Adding it because I spent an afternoon working it out and nobody should have to twice.

0 likes in reply to #20 11mo
NV
n.vukovicTL23 Sep 2025#23

Worth separating two things that post #19 runs together.

A treatment-policy estimand asks what happens to people assigned to a strategy, including those who abandon it. A hypothetical estimand asks what would have happened had everyone continued. Both are legitimate and they give different numbers.

15 likes 11mo
AD
appeals_deskTL3Regular6 Sep 2025#24

Post #23 is right about the mechanism and I think understates the practical bit.

Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting.

5 likes 11mo
YR
y.rahimiTL28 Sep 2025 · edited#25
p.diallo, post #2: Risk of bias: structured appraisal of internal validity. Key things to assess: randomisation method (was it truly random or could someone predict the next assignment), concealment (could randomisation be subverted), blinding (who was blinded and why or why not), completeness of outcome reporting. Go to post

Coming back to post #23, because the follow-up matters more than the original answer.

Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit.

0 likes in reply to #2 11mo
P
preregisteredTL3Research methods11 Sep 2025#26

Absolute and relative effects answer different questions. Write down the event rate in each arm and the difference between them; everything quotable is derived from those two numbers.

A guess, clearly labelled as one.

30 likes 11mo
RP
r.petrovTL213 Sep 2025#27

A treatment-policy estimand asks what happens to people assigned to a strategy, including those who abandon it. A hypothetical estimand asks what would have happened had everyone continued. Both are legitimate and they give different numbers.

I would hold that lightly until someone with a larger sample weighs in.

10 likes 10mo
PE
ppm_errorTL3Analytical chemist15 Sep 2025#28

Adding a note of thanks rather than an opinion. I did not know most of that.

3 likes 10mo
BV
b.vanheckeTL218 Sep 2025#29
dr_okonkwo, post #11: Absolute and relative effects answer different questions. Write down the event rate in each arm and the difference between them; everything quotable is derived from those two numbers. Go to post

I read post #27 twice before replying, because I had assumed the opposite.

Absolute numbers, not just relative: a 30% relative reduction tells you the ratio but not the practical magnitude. The event rate in each arm and the difference between them tells you how many people benefit.

It cost nothing to check and would have cost something not to.

5 likes in reply to #11 10mo
C
chromatogramTL4Analytical chemist20 Sep 2025#30

This follows post #27 rather than contradicting it.

Discontinuation handling is the methodological detail that most changes a result and gets the least attention. Read how missing data was imputed before reading the effect size.

0 likes 10mo