Declaration: 
Perform2SampleTTest (Data1, Data2: TDoubleArray; TestType: TtTestVersion; OneSided: boolean; var Mean1, Mean2, Stdv1, Stdv2, MeanDiff, StdDiff, tStatistic: double; var df: integer): double; 
The function Perform2SampleTTest returns the probability associated with a 2sample ttest (probability which indicates the likelihood that two samples have come from the same two underlying populations having the same mean).
The parameters Data1 and Data2 contain the data. Note that the number of elements of the two data arrays have to be equal if a paired ttest is to be performed. The parameter TestType determines the type of ttest to be performed. The parameter OneSided has to be TRUE for onesided tests, or FALSE for twosided tests.
The function returns additional parameters obtained during the ttest calculation:
Mean1, Mean2 
means of the two samples 
Stdv1, Stdv2 
standard deviations of the two samples 
MeanDiff 
mean of paired differences (only available if TestType is ttPaired) 
StdDiff 
standard deviation of paired differences (only available if TestType is ttPaired) 
tStatistic 
t statistic 
df 
degrees of freedom 

Hint 1: 
If the TestType is set to ttHeteroScedastic the Welch approximation is used yielding probabilities which are slightly different to the results compared to the results of Microsofts Excel. The slightly different probabilities have no practical consequences since the Welch approximation is (too) conservative in most instances. 
