Equations for Human Factors


Weber's Law

k = 

   DI    
     I

 

Fechner's Law

                                                S=klog(I)

Steven's Power Law
                                                S=cIb

Error Probability (EP) or Human Error Probability (HEP)

                                            HEP = EP = (# errors/# opportunities)

Reliability

                                    R= (# successes/# opportunities)=1-EP
                                    Of Components in serial       R = P Ri
                                            Of Components in parallel    R = 1-(1-Ri)c 
                                           (note: this equation assumes that each component in parallel has the same reliability)

Calculation of decibels

                               db = 10Log(P12/Pr2)
                                        or
                                     db = 20Log(P1/Pr) 

                          
 
Calculations of Measures Related to Visual Intensity:
  

Reflectance = 

Luminance  
Illuminance

 
          Contrast Ratio                    

                            CR = 

   Lmax
_____________
   Lmin

 
 Visual Angle

                                                    a = tan-1(S/D)

        S = size of object, D = distance to object. 
 

Color Coordinates from Tristimulus Values

                                   

x=        SX        
 SX+SY+SZ
y=        SY        
 SX+SY+SZ

Fitt's Law

Index of Difficulty

The Law

 

To convert from Log10 to Log2, multiply by 3.32.