Mid rank

If x1,x2,..xn are ‘n’ observations in a data set with frequencies f1,f2,…fn respectively. Lety1,y2,…,yn be the observed data arranged in the ascending order with respective ordered frequencies g(1),g(2),…g(n). Then mid-rank for yi is then calculated by
Ri=g(1)+g(2)+…+g(i-1)+(g(i)+1)/2;where i=1,2,…,n