Jd Analysis - Xls Download

Hi,
Can anyone help me to find out how to measure the importance of a task or a knowledge within a porfolio of multitask ...factors should have in a job description.
Enlosed here an example.
Please help!
Thank a ton :-D

Dear Binababu,
I really appreciated your quick response in my query. Thanks much. Very good source you referred me to read on. However, what i concern here is how they can measure the importance of each task. it should have a formulation.
let's take a look below detailed description for typical HRM position
%importance Category Task
79% Core Administer compensation, benefits and performance management systems, and safety and recreation programs.
78% Core Identify staff vacancies and recruit, interview and select applicants.
76% Core Allocate human resources, ensuring appropriate matches between personnel.
73% Core Provide current and prospective employees with information about policies, job duties, working conditions, wages, opportunities for promotion and employee benefits.
so how to know 79%, 78%, 76% ....what i really want to find out is the formulation to define these number
Please help