The T-test formula described uses the difference between means as its numerator. Which expression represents this?

• A. Y mean - X mean
• B. X mean - Y mean
• C. Y mean + X mean
• D. Y mean / X mean

Answer: (A)

In a t-test, the part of the formula that measures how far apart the groups are is the difference between the two sample means. Using the given notation, that difference is obtained by subtracting the mean of X from the mean of Y: Y mean minus X mean. The sign of this value indicates which group has the higher mean. Adding the means would not represent separation, and dividing would form a ratio rather than a difference, so they don’t quantify how far apart the means are.