How to Use Excel's TTEST Function. Excel's TTEST function calculates the probability that two samples are from the same population, assuming the populations have the same mean. TTEST returns the probability of a Student's t-Test, a method of testing a hypothesis made about the mean of a sample population. It is named after William Sealy Gosset who published the first paper on the subject in 1908 under the pseudonym "Student." Here's how to use Excel's TTEST function.

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Learn the syntax for TTEST. It is TTEST (array_1, array_2, tails, type).

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Provide array_1 and array_2 as the data sets. TTEST will use this data to compute a nonnegative t-statistic. TTEST will return #N/A if these data sets have different sizes and the type is 1 (paired t-Test.)

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Use tails to indicate the desired number of distribution tails. TTEST will return #NUM! if tails is a number other than 1 or 2. TTEST will return #VALUE! if tails is nonnumeric.

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Input a value of 1 for tails to use a one tailed distribution. TTEST will return the probability of a higher t-statistic, assuming that the populations represented by array_1 and array_2 have the same mean. A value of 2 will use a two-tailed distribution that returns a value twice as high as the one returned for a one-tailed distribution. This value is the probability that the absolute value of the t-statistic is higher using the "same population means" assumption.

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Specify type as the type of t-Test to perform. A type of 1 indicates a paired t-Test, a type of 2 is a two-sample equal (homoscedastic) variance and a type of 3 indicates a two-sample unequal (heteroscedastic) variance. TTEST will return #VALUE! if type is nonnumeric.