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From |
Maarten buis <maartenbuis@yahoo.co.uk> |

To |
statalist@hsphsun2.harvard.edu |

Subject |
Re: st: Looping through observations |

Date |
Tue, 23 Nov 2010 09:03:32 +0000 (GMT) |

--- On Mon, 22/11/10, Laurie Molina wrote: > The data set is about the time required achive a desired > amount of saving. For each observation in the data set, i > have income, and saving rate. But future income depends > on the saving rate. I am guessing you are dealing with a regular compound interest problem. Income is confusing terminology in that case, so I change that to value (v). To make explicit that v is a function of time I use v_t. I assume that there are three knowns: the initial savings (v_0), the goal (v_g), and the interest rate (i), and that you want to know how long it takes to reach that goal (t) given those three knowns. Start with writing out how the value at time t depends on v_0, i, and t: v_t = v_0*(1+i)^t Set v_t equal to v_g: v_g = v_0*(1+i)^t Take the logarithm of both the left hand side and the right hand side: ln(v_g) = ln(v_0*(1+i)^t) Remember some basic rules for dealing with logarithms: ln(a*b) = ln(a) + ln(b) ln(a^b) = b*ln(a) So: ln(v_g) = ln(v_0) + t* ln(1+i) Now you can see that you can compute t without resorting to a loop: t = [ln(v_g) - ln(v_0)]/ln(1+i) In Stata you can implement that using a regular -generate- command: *----------- begin example -------------- // create some data drop _all input v_0 v_g i 20 100 .02 500 1000 .015 100 200 .02 end // compute the time till the goal is reached gen t = (ln(v_g) - ln(v_0))/ln(1+i) // display the results list *----------- end example ---------------- (For more on examples I sent to the Statalist see: http://www.maartenbuis.nl/example_faq ) Hope this helps, Maarten -------------------------- Maarten L. Buis Institut fuer Soziologie Universitaet Tuebingen Wilhelmstrasse 36 72074 Tuebingen Germany http://www.maartenbuis.nl -------------------------- * * For searches and help try: * http://www.stata.com/help.cgi?search * http://www.stata.com/support/statalist/faq * http://www.ats.ucla.edu/stat/stata/

**References**:**st: Looping through observations***From:*Laurie Molina <molinalaurie@gmail.com>

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