Actuarial Outpost Not Understanding This Mortality Formula
 Register Blogs Wiki FAQ Calendar Search Today's Posts Mark Forums Read
 FlashChat Actuarial Discussion Preliminary Exams CAS/SOA Exams Cyberchat Around the World Suggestions

 Long-Term Actuarial Math Old Exam MLC Forum

#1
02-21-2019, 10:28 PM
 SweepingRocks Member SOA Join Date: Jun 2017 College: Bentley University Posts: 122
Not Understanding This Mortality Formula

https://imgur.com/a/bFdEfxA

Here's the thing I'm having an issue with. I understand that the force multiplied by the probability of survival is equal to the probability of death at the time of the force.

I don't understand the jump from the second line to the third line. Wouldn't tP'(1)x * ux+t be equal to tq'(1)x? Why are we jumping to the conclusion that it's a constant q'(1)x? The video just says it's constant because UDD, but I'm having trouble making the connection.

__________________
FM P MFE STAM LTAM in April

Former Disney World Cast Member, currently no idea what I'm doing

"I think you should refrain from quoting yourself. It sounds pompous." - SweepingRocks
#2
02-21-2019, 10:54 PM
 Academic Actuary Member Join Date: Sep 2009 Posts: 8,572

Factoring out the q' requires the UDD assumption. Under UDD deaths are at a constant rate over the year. That constant rate is q'x.
#3
02-21-2019, 11:10 PM
 SweepingRocks Member SOA Join Date: Jun 2017 College: Bentley University Posts: 122

Quote:
 Originally Posted by Academic Actuary Factoring out the q' requires the UDD assumption. Under UDD deaths are at a constant rate over the year. That constant rate is q'x.
Okay back up. Still confused. I thought tP'x * u(x+t). Would be the rate of deaths during the period x to x+t. If t<1, I get that the rate would be constant, but I don't see how we get the annual death rate q'x, rather than t*q'x.

In other words, I don't see how if we're taking someone surviving t years (t<1), then they die at time t, then saying that's the same as the chances of someone dying in the first year.
__________________
FM P MFE STAM LTAM in April

Former Disney World Cast Member, currently no idea what I'm doing

"I think you should refrain from quoting yourself. It sounds pompous." - SweepingRocks
#4
02-21-2019, 11:25 PM
 Academic Actuary Member Join Date: Sep 2009 Posts: 8,572

It's more intuitive to think of a cohort than an individual. Lets there are 1000000 lives at the beginning of the year with lets say q' = .02. The lives are dying at the rate of 20000/year with the number of deaths in any time interval proportional to the time interval. Over the year the number of survivors is decreasing by the force of mortality increases exactly enough to keep the rate of deaths constant.

UDD is equivalent to the product of the probability of survival and the force of mortality is constant.
#5
02-21-2019, 11:30 PM
 SweepingRocks Member SOA Join Date: Jun 2017 College: Bentley University Posts: 122

Quote:
 Originally Posted by Academic Actuary It's more intuitive to think of a cohort than an individual. Lets there are 1000000 lives at the beginning of the year with lets say q' = .02. The lives are dying at the rate of 20000/year with the number of deaths in any time interval proportional to the time interval. Over the year the number of survivors is decreasing by the force of mortality increases exactly enough to keep the rate of deaths constant. UDD is equivalent to the product of the probability of survival and the force of mortality is constant.
That actually makes so much sense! Thank you very much!
__________________
FM P MFE STAM LTAM in April

Former Disney World Cast Member, currently no idea what I'm doing

"I think you should refrain from quoting yourself. It sounds pompous." - SweepingRocks