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```I additionally have 7 other variables a, b, c, d, e, f, g that I would like
to add to the original model in all possible combinations ie:

original model or a
original model or b
original model or c
original model or d
original model or e
original model or f
original model or g
original model or a or b
original model or a or c
.....
original model or a or b or c
original model or a or b or d
.....
original model or a or b or c or d or e or f or g

I am scratching my head trying to work out how to include this loop within
the original loop and would be grateful for any suggestions.

Do file so far is:
[r12cough and rrate1 are two of the seven additional variables I am adding
in combination to the original model and are shown as an example of what I
am trying to achieve... but by using a loop]

xi:logistic died i.sex1 i.r18unwel_1 i.r21dyspn i.r25whtl i.hiv i.pulse1

*generates a variable p which is the probability of each individual being a
case, based on this logistic regression model
predict p
*generates a variable x which is going to be used as the mortality status
based on the model
gen x_reg=0
gen y_reg=0
gen x_h=0
gen y_h=0
qui gen n=1

*Creates a loop to do the sens/spec/ppv/npv for cutoffs from 0.005 to 0.650
- incrementing by 0.005

forvalues i=0.005(0.005)0.65 {
*resets x to zero within each value of cutoff
qui replace x_reg=0
qui replace y_reg=0
qui replace x_h=0
qui replace y_h=0
disp ""
disp ""
disp "cutoff = " `i'
*replaces x=1 if the individual has a probability of being a case which is
higher than the cutoff i
qui replace x_reg=1 if p>=`i' & p!=.

qui replace x_h=1 if (p>=`i' & p!=.)
qui replace x_h=0 if p<`i'

qui replace x_h=1 if (p>=`i' & p!=.) | r12cough==1 | rrate1==1
qui replace x_h=0 if p<`i' & r12cough!=1 & rrate1!=1

*tabulates true mortality status (died) against predicted (x) for each
cutoff value
disp "True vs predicted mortality status based on LR model only"
tab died x_reg, row
disp "True vs predicted mortality status based on LR model plus r12cough or
rrate1"
tab died x_h, row

qui replace y_reg=1-x_reg
qui replace y_h=1-x_h
preserve
qui collapse (sum) x_reg y_reg x_h y_h n, by(died)
qui gen sensspec_reg=x_reg/n if died==1
qui gen sensspec_h=x_h/n if died==1
qui gen id=1
qui replace sensspec_reg=y_reg/n if died!=1
qui replace sensspec_h=y_h/n if died!=1
qui reshape wide sensspec_reg x_reg y_reg sensspec_h x_h y_h n, i(id)
j(died)
qui gen lr_reg=sensspec_reg1/(1-sensspec_reg0)
qui gen ppv_reg=x_reg1/(x_reg0+x_reg1)
qui gen npv_reg=y_reg0/(y_reg0+y_reg1)
qui gen lr_h=sensspec_reg1/(1-sensspec_h0)
qui gen ppv_h=x_h1/(x_h0+x_h1)
qui gen npv_h=y_h0/(y_h0+y_h1)
disp "Sensitivity based on LR model only = " %5.3f sensspec_reg1  " at
cutoff p="  `i'
disp "Specificity based on LR model only = " %5.3f sensspec_reg0  " at
cutoff p="  `i'
disp "Positive predictive value based on LR model only = " %5.3f ppv_reg  "
at cutoff p="  `i'
disp "Negative predictive value based on LR model only = " %5.3f npv_reg  "
at cutoff p="  `i'
disp "Likelihood ratio based on LR model only = " %5.3f lr_reg  " at cutoff
p="  `i'
disp "Sensitivity based on LR model plus r12cough or rrate1= " %5.3f
sensspec_h1  " at cutoff p="  `i'
disp "Specificity based on LR model plus r12cough or rrate1= " %5.3f
sensspec_h0  " at cutoff p="  `i'
disp "Positive predictive value based on LR model plus r12cough or rrate1h=
" %5.3f ppv_h  " at cutoff p="  `i'
disp "Negative predictive value based on LR model plus r12cough or rrate1= "
%5.3f npv_h  " at cutoff p="  `i'
disp "Likelihood ratio based on LR model plus r12cough or rrate1= " %5.3f
lr_h  " at cutoff p="  `i'

restore
}

Many thanks,

Peter MacPherson
Liverpool School of Tropical Medicine

Dr Peter MacPherson MBChB MPH
Wellcome Trust Clinical Research Fellow
PhD Candidate

Apt 5, 14 South Albert Road
Liverpool, L17 8TN
United Kingdom

Mob: +447519592227
email: petermacpherson@mac.com
skype:petermacpherso

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