Vpo function

Variance for estimated Mann-Whitney parameter under proportional odds.

A function to calculate the variance of the estimated Mann-Whitney parameter under the proportional odds model.

Vpo(PHI, tf, ny, nx)

Arguments

• PHI: Mann-Whitney parameter
• tf: tie factor
• ny: sample size for the y variable
• nx: sample size for the x variable

Details

Uses uniroot to find the variance numerically. The approximation using the LAPH method (Lehmann alternative model combined with the proportional hazards model) is much faster and is the default.

Returns

the variance of the estimated Mann-Whitney parameter

Examples

Vpo(.7,.8,5,7)
## Compare to LAPH method
VLAPH<- function(PHI,tf=tiefactor,ny=n.y,nx=n.x){
        tf*(PHI*(1-PHI)/(ny*nx))*
          (1+((ny+nx-2)/2)*
             ((1-PHI)/(2-PHI)+PHI/(1+PHI)))
      }   
VLAPH(.7,.8,5,7)