Simple math formula to calculate average

I am making a software that deals with employee trust. I am trying to make a math formula (no need to tell that I am bad in math :) ).

Here is the scenario:

Employee 1 trusts employee 2 = > 20% or 0.2 (average trust of employee 1 with employee 2)
Employee 2 deals with 10 customers => 13%  (average - 10 customers trust on Employee 2)
Employee 1 also deals with 7 customers that customers belongs to Employee 2's 10 customers. => 37% ( average 7 customers trust on employee 1)

Employee 1 dealt with 10 customers for last 17 years.
Employee 2 dealt with 7 customers for last 1 years.

Now I want to calculate that how can we say that Employee 1 is much trust worthy than Employee 2? If I just see average than I can say Employee 1 but he only dealt with customers for 1 year. Now I want to make a generic formula to see which one is more trust worthy and I want to use all three values that are ( employee to employee, employee to customer, and number of year) to calculate average percentage. I want to increase or decrease an employee trust based on these values. Please remember I want to decrease as well for some employee I can not say that all employee or 90+ employee are trust worthy etc.

What I did: I just simply plus employee to employee and employee to customer tr开发者_运维问答ust weight

Employee 1 = 0.57
Employee 2 = 0.33

I don't know what to do for years.

Current system just calculate employee to employee trust that is not good enough to trust itself.

I am not restricted to any programming language because I just want to make a generic formula. All ideas and suggestions are most welcome !

If you think my problem relates to any previously developed trust model then please do let me know (I already know web trust model but I don't know how to fit that in my situation)

You want a logarithmic function for this.

y = k*log(t+b)

t is the time they've maintained their customer relations in some useful unit of time (years, months, etc.), k and b are constant factors determined experimentally. Add or multiply the result of this equation to what you have so far.

I don't think trust can be simply averaged - it's a serious area of research. Here's a typical paper: http://www.cs.uwaterloo.ca/~rckerr/KerrCohen-pst06.pdf which uses considerably non-linear functions.