v0.1.1 (powerMediation) April 26, 2013
  (1) add functions 'powerLogistic.OR' and 'SSizeLogistic.OR' to calculate
      power and sample size for logistic regression
v0.1.0 (powerMediation) Nov. 28, 2012
  (1) fixed 2 bugs in the file: power_VSMc_linear.R 
v0.0.9 (powerMediation)
  (1) fix a typo in man file for 'powerMediation.Sobel'

v0.0.8 (powerMediation)
  (1) Dr. George D. Papandonatos (gdp@stat.brown.edu)
      sent an e-mail to me today and kindly mentioned a few improvements
      for power calculation for Sobel test:

#############################################
Dear Weiliang,

your help file for deriving the power of the Soble test allows
users to enter sigma.m, sigma.x, theta.1a, and sigma.e as free
parameters. However,

sigma.m^2 = theta.1a^2*sigma.x^2+sigma.e^2

so that sigma.m is not a free parameter. Indeed, in the example
given in your documentation, it appears that

sigma.m = sqrt( .1701^2*.57^2+.20^2) = 0.22

which conflicts with the 0.612 value assigned to this parameter.
I belive that you should not have sigma.m as an argument to this
function.

Further, the documentation refers to sigma.m, sigma.x as variances,
whereas your code suggests they are standard deviations.

Also, you refer to sigma.theta and sigma.lambda as standard deviations,
whereas the formulae given suggest that they are variances.

#############################################


v0.0.7 (powerMediation)
  (1) fixed a bug in calculating sample size/power for mediation effect
      based on Vittinghoff, Sen and McCulloch's (2009) method: corr.xm
      should be corr.xm^2 in the formula
  (2) fixed a bug in citing author names in R code or man file: the author 'Sen' was accidentally dropped.
  (3) added sample size/power calculation for logistic, poisson, and cox regression based on Vittinghoff, Sen and McCulloch's (2009) method. Thanks for Dr. Rui Wang who pointed out the sample size/power calculation formula for these 3 addition models in Vittinghoff, Sen and McCulloch (2009) and encouraged me to implement them. She even sent to me an sample R code for sample size calculation for logistic regression  for testing mediation effect.
  (4) replaced the examples in functions based on Vittinghoff, Sen and McCulloch's (2009) method

