# Talk:Force of mortality

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Shouldn't it be: ${\displaystyle P(xx)=P(x?

This gives:

${\displaystyle p_{\Delta x}(x)=P(xx)={\frac {F_{X}(x+\Delta \;x)-F_{X}(x)}{1-F_{X}(x)}}.}$

The force of mortality is not the probability that those surviving until age ${\displaystyle x}$ die at age ${\displaystyle x}$. It is the rate of change of this probability:

${\displaystyle \mu (x)=\lim _{\Delta x\rightarrow \infty }{\frac {p_{\Delta x}(x)}{\Delta x}}={\frac {F_{X}(x+\Delta \;x)-F_{X}(x)}{\Delta x(1-F_{X}(x))}}}$.

Force of mortality should be proportional to the rate of change of the mortality rate, so this restating seems logical. Thus the taking the derivative with respect to ${\displaystyle x}$ of the mortality rate(probability of death) at age ${\displaystyle x}$ would give the force of mortality at age ${\displaystyle x}$. Think along lines of physics where the word 'force' is more or less defined: ${\displaystyle F=ma}$, with the mass being 1. Or maybe the mass should be thought of as the inverse of the probability of surviving until age x.

It seems that this has been the way it appeared in previous versions of the article. What the differential in the denominator meant must have not been made clear in those earlier revisions. I hope my argument will be seen as transparent.

The issue is that it is not exactly clear where the ${\displaystyle \Delta x}$ in the denominator comes from. I will make this change to the webpage if I get no objections in some reasonable time period (2-4 weeks?).

## Article Lacks Introduction/Explanation

It's always frustrating when I come across articles such as this one. It completely lacks a non-technical, written introduction and description. 86.10.244.90 (talk) 10:27, 18 June 2015 (UTC)