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Copyright © 2012, Campbell R. Harvey. All Rights Reserved.

Half Life

In a pool of mortgages underlying a mortgage-backed security, the length of time it takes for half the aggregate principal to be repaid. A half life is normally about 12 years, but this may vary depending on how often homeowners refinance their mortgages (which pays off all principal at once). Half lives are calculated for mortgage-backed securities issued or guaranteed by Freddie Mac, Fannie Mae, and Ginnie Mae.

Weighted Average Life

The amount of time for the principal on a loan or a mortgage to be paid off. The length of the weighted average life depends on the amount of principal paydowns and how often they are made. Once the WAL is calculated, one can determine how long it will take to pay off half of the remaining principal.
Farlex Financial Dictionary. © 2012 Farlex, Inc. All Rights Reserved


The length of time before half the principal on a debt is expected to be repaid through amortization or sinking fund payments. For example, a 25-year bond issue may require the issuer to retire 5% of the beginning principal commencing 5 years after the issue date. Thus, the bond issue has a half-life of 5 years plus the number of years required to retire half the issue, or 15 years. Mortgage-backed securities often have a relatively short half-life because many homeowners pay off or refinance their mortgages early. Also called average life.
Wall Street Words: An A to Z Guide to Investment Terms for Today's Investor by David L. Scott. Copyright © 2003 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved. All rights reserved.
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Halflife: Improvisations and Interviews, 1977-1987.
Epoetin alfa and epoetin beta are approved rhEPO analaogues, with plasma halflife of 6-8 hours.
Among the more thoughtful reviews was one by Meghan O'Rourke, Slate's literary editor and the author of Halflife, though even she found that the poems are less interesting as works of art than in the way they "restore individuality to those who have been dehumanized and vilified in the eyes of the public."
(13) Given that dopamine agonists have a halflife of several hours, decreased motor complications may be realized in patients with early PD who utilize these agents in combination as opposed to sole use of levodopa.
He recommends using [D.sub.3] to raise vitamin D levels because it has a longer halflife than does [D.sub.2] For patients with vitamin D deficiency, he uses 50,000 IU per week.
The poets who made this conclusion are Meghan O'Rourke, an editor for Slate and the Paris Review and author of Halflife; J.
With clomiphene citrate, depletion of estrogen receptors has long-term effects because of the drug's relatively long halflife (several days).
It has a short halflife (15-20 minutes), being rapidly cleared by the liver.
If, as supporters of reprocessing advocate, two fission products with a 30-year halflife, cesium-137 and strontium-90, are separated and stored on the surface, the remaining fission products from perhaps 100 times as much spent fuel could be stored in the mountain.
For a first-order process, the halflife can be calculated by the following equation:
Technically, [sup.41]Ca is a radioactive isotope, but its halflife is so long that, for all practical purposes, it behaves as a stable isotope.