st: difference between estimated ratios of x/y and ax/ay

["Zurab Sajaia" <zsajaia@hotmail.com>]

Dear All,

I was estimating ratio of two variables (in fact, svy : ratio, but problem 
is the same for both) and after noticing that both numerator and denominator 
contained same component I decided to check whether canceling it would 
affect the results. To my surprise it did.  Lets consider this example:

sysuse auto, clear

. ratio weight / length
...
   _ratio_1 |   16.06673   .2807194      15.50726     16.6262

...

.generate double wa = weight * trunk
.generate double la  = length * trunk

.ratio wa / la
...
   _ratio_1 |   16.48413   .2797573      15.92658    17.04169
...


Have anybody experienced similar case? does this thing come purely from 
numerical, rounding issues or there is something bigger (I can't think of 
any though)?

Any insight would be appreciated.
Thanks
Zurab 
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