[EpiData-list] How To: Compare treatment A and B, when not given on all days for all patients

epidata-list at lists.umanitoba.ca epidata-list at lists.umanitoba.ca
Fri Apr 25 03:12:00 CDT 2008


For a medication study we wish to test the hypothesis of no diffence in
use of additional morphine for patients given two different pain regimens.

The data collected for each patient is the date and time of additional
medication from day one until day seven.  The number of additional
medication given varies from 0 to many. Therefore we record the data as:

id   : patient id
d  : date of medication
m  : Dosage of the medication

Basic data are saved for each patient
grp : treatment group, A vs. B
dayop : Date of surgery

For the analysis we also need:
variable m   : sum of additional morphica given
varable d    : day of treatment

This would seem easy:
read medication

* add day of surgery etc.:
merge id /table=basicdata
gen i day = d - dayop

* now aggregate on day of medication
aggregate id day grp /sum=m

* save for later:
savedata daymedication /replace

* this creates the summary variable summ for m
* summ contains the sum of medication on each day for each patient

means summ /by=grp      /t       : overall
means summ /by=grp / t if day = 1    : example comparison on day one.

But this will give a wrong result, since only persons given medication are
recorded in the medication file.

We therefore need to restructure the analysis:

A. Create a complete file for all patients:
* generate file of patient id' numbers
* (here 22 patients on day one and two):
generate 44
gen i idn = recnumber +1

* we need two lines for each patient:
gen i id = (idn div  2)
gen i grp = 1

* our dataset is such that first 11 patients are group 1 and remaining
group 2:
if id > 11 then grp = 2
gen i day = 1
if  (id = id[recnumber-1]) then day = 2
drop idn

labeldata "Key file for all patients"
savedata key /replace
* this file now contains exactly one record for all patients on each of
day one and two.

B now we can analyse the data again:
    (notice the daymedication file created above)

* get the complete file:
Read key /close

* add medication information:
merge id day /table=daymedication

* add basic information on
merge id /table=basicdata

* for patients with no medication summ is equal to . (missing)
recode summ .=0

* now summ contains the sum of medication on each day for each patient

* show the observations for all days:
* generate indicator variable based on day and group
gen i g = 10*(grp-1) + day
labelvalue g /1="Case Day 1" /2="Day 2"  /11="Control day 1" /12="Day 2"

* verify distribution of summ as a probit plot
cdfplot summ /p

* for aggregation in the cdfplot the variable must be integer:
gen i mx = integer(summ*1000)
cdfplot mx /p /agg

* for my specific data showing a straight line
* (with a slight "too many at zero" problem)

* show as box and dotplot:
box summ /by=g  /n  /out
dotplot summ /by=g /sizex=600 /xa /di=0.25 /ti="Amount given - patient level"

* finally test the hypothesis
means summ /by=grp      /t       : overall
means summ /by=grp if day = 1    : comparison on day one.
means summ /by=grp if day = 2    : comparison on day two.


Jens Lauritsen
Coordinator and initiator of EpiData Project
http://www.epidata.dk




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