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Re: st: RE: RE: generate graph (try 2)


From   Nikolaos Pandis <npandis@yahoo.com>
To   "statalist@hsphsun2.harvard.edu" <statalist@hsphsun2.harvard.edu>
Subject   Re: st: RE: RE: generate graph (try 2)
Date   Thu, 3 Nov 2011 03:51:28 -0700 (PDT)

Dear Phil,

The first piece of code worked well, the second had a minor issue on the twoway part  probably from copy/paste etc-I fixed it no problem

On a more careful reading of the code, although the graphs have a similar look to what want, I am wondering if this is not the graph I need.

On the x-axis I would like to show the p-value and how the p-value decreases for a given effect/sd (effect/sd remain constant) as we increase the sample size. The idea is to show how interpreting trial results from p-values may be misleading as you may have an uniportant clinical effect which may be statistically significant or nonsignificant depending on the sample size used.
 
If I unerstood correctly the code you supplied indicates how the sample size requirements decrease as the a-level changes?

Please advise.

Many thanks,

Nick

 


----- Original Message -----
From: Philip Ryan <philip.ryan@adelaide.edu.au>
To: statalist@hsphsun2.harvard.edu
Cc: 
Sent: Thursday, November 3, 2011 9:48 AM
Subject: st: RE: RE: generate graph (try 2)

<<>> 

For reasons not clear to me, the return from my own initial posting seemed
to have left out some carriage returns that were present in what I sent.
Perhaps this, inserting several double carriage returns, will be better. (Of
course, code should be run from the do file editor).

*==== begin code ====*

clear
tempname temp_ss
local sd .75
local pow 0.8
local m1 0.35
local m2 0

quietly {

postfile `temp_ss'  sample_size  alpha  using  my_ss, replace

forvalues alpha = .01(.01).2{

sampsi `m1'  `m2', alpha(`alpha')  power(`pow') sd(`sd')  r(1) 

post `temp_ss' (r(N_1)) (`alpha') 

} 

postclose `temp_ss'

} 

/* now make use of our new data set */

use my_ss
list, clean abbrev(16)
graph twoway line  sample_size alpha,  ///  ytitle("sample size in each
group") xlabel(0(.01).2) ///
lwidth(medthick) lcolor(blue)

*=== end code ===



Philip Ryan
Professor and Director
Data Management & Analysis Centre
School of Population Health & Clinical Practice
University of Adelaide

Postal address:
DMAC
Mail Drop DX650 511
University of Adelaide 5005
South Australia
Australia

location:
Level 6
Bice Building
Royal Adelaide Hospital
North Terrace
Adelaide SA

tel: +61 (0)8  83033570
fax: +61 (0)8 82234075


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Philip Ryan
Sent: Thursday, 3 November 2011 6:04 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: RE: generate graph

<<>>

Something like this (grafting onto -sampsi-) ?:

*==== begin code ====*

clear
tempname temp_ss
local sd .75
local pow 0.8
local m1 0.35
local m2 0

quietly {
postfile `temp_ss'  sample_size  alpha  using  my_ss, replace

forvalues alpha = .01(.01).2{
sampsi `m1'  `m2', alpha(`alpha')  power(`pow') sd(`sd')  r(1)
post `temp_ss' (r(N_1)) (`alpha')
} 

postclose `temp_ss'

} 

/* now make use of our new data set */

use my_ss
list, clean abbrev(16)
graph twoway line  sample_size alpha,  ///
ytitle("sample size in each group") xlabel(0(.01).2) ///
lwidth(medthick) lcolor(blue)

*=== end code ===


Philip Ryan


-----Original Message-----
From: owner-statalist@hsphsun2.harvard.edu
[mailto:owner-statalist@hsphsun2.harvard.edu] On Behalf Of Nikolaos Pandis
Sent: Thursday, 3 November 2011 5:34 PM
To: statalist@hsphsun2.harvard.edu
Subject: st: generate graph

Hi to all.
 
I was wondering if someone could help me generate the following graph:
 
twoway line graph showing on the y-axis: sample size and on the x-axis:
p-value.
 
The objective is to show how the p-value decreases (increases) as the sample
size increases (decreases) given the same effect size and sd
 
Many thanks,
 
Nick

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