TTEST function

Returns the probability associated with a Student's t-test


 * TTEST(array1,array2,tails,type)
 * Array1 - the first data set.
 * Array2 - the second data set.
 * Tails - specifies the number of distribution tails.
 * If tails = 1, TTEST uses the one-tailed distribution.
 * If tails = 2, TTEST uses the two-tailed distribution.
 * Type - the kind of t-Test to perform.
 * If type equals 1, a paired test is performed
 * If type equals 2, a two-sample equal variance (homoscedastic) test is performed.
 * If type equals 3, a two-sample unequal variance (heteroscedastic) test is performed.
 * If array1 and array2 have a different number of data points, and type = 1 (paired), TTEST returns the #N/A error value.
 * The tails and type arguments are truncated to integers.
 * If tails or type is nonnumeric, TTEST returns the #VALUE! error value.
 * If tails is any value other than 1 or 2, TTEST returns the #NUM! error value.
 * TTEST uses the data in array1 and array2 to compute a non-negative t-statistic.
 * If tails=1, TTEST returns the probability of a higher value of the t-statistic under the assumption that array1 and array2 are samples from populations with the same mean.
 * The value returned by TTEST when tails=2 is double that returned when tails=1 and corresponds to the probability of a higher absolute value of the t-statistic under the “same population means” assumption.