##########################################################
# df is a function for calculating the degree of freedom #
# for the 2 independent sample t-test (Formula 7.1 in    #
# the textbook).                                         #
# Needs the standard deviation in both samples, as well  #
# as the 2 sample sizes.                                 #
##########################################################

df = function(sd1,sd2,n1,n2){
  (sd1^2/n1+sd2^2/n2)^2/(sd1^4/(n1^2*(n1-1))+sd2^4/(n2^2*(n2-1)))
}

# example of use: if standard deviations are sd1=1.7 and sd2=2.3
# and sample sizes are n1=8 and n2=10, then do
# df(1.7,2.3,8,10)