st: Simulating time series with if else condition

[mweberru@students.mail.uni-mannheim.de]

Dear Statalist,

I would like to generate a time series of debt ratios using Monte  
Carlo Simulation for ~ 200 firms over 19 years. I start with the  
debt-level (in $) in 1971 and add debt-issues/-repayments (in $,  
determined via OLS regression with a funds flow deficit as independent  
variable) to generate next year's debt-level. This process is repeated  
until 1989, thus generating a series of debt-ratios for each firm from  
1971 to 1989.

The firm is assumed to issue debt if the funds-flow deficit is  
positive and to repay debt if the deficit is negative. However, debt  
is only repayed until the debt-level has reached 0.

I have written a program for this, using the "postfile-"command".  
However, I am having troubles with adding the condition that the  
minimum debt level cannot be less than zero. Thus, the problematic  
part of the following program is the if-else condition.

I would be really thankful if anyone of you could help me solve my problem.
Thanks in advance for your help!

Regards,
M.Weber


Here is the do-file:

set seed  75225379

scalar const=0
scalar coeff=0.7

gen u=.
gen x=.
gen debtissue_sim=.
by id: gen debtlevel_sim=.
gen debtratio_sim=.

postfile sim  debtissue_sim   debtlevel_sim   debtratio_sim   using    
sim_results, replace
quietly {
forvalues i=1/1000 {
tsset id fyear
replace u=rnormal()
replace x=def                                                           
              /*def: funds flow deficit*/
replace debtissue_sim=const + coeff * x   + u                         
/*estimating the level of debt-issue/-repayments*/
regress debtissue_sim  x
if fyear==1971{
    by id: replace debtlevel_sim=actual_debtlevel                 
/*start with actual debt-level in 1971*/
    }
else if fyear>1971 & (L.debtlevel_sim+debtissue_sim)<=0 {        /*in  
years following 1971: no negative debt-level*/
    by id: replace debtlevel_sim=0
    }
else {
    by id: replace debtlevel_sim=L.debtlevel_sim+debtissue_sim     
/*transform debt-level to debt-ratio*/
    }
replace debtratio_sim=debtlevel_sim/net_assets
post sim (debtissue_sim) (debtissue_sim_at) (debtlevel_sim) (debtratio_sim)
}
}
postclose sim




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