The test statistic for testing the difference between two means are valid only when the sampling distribution of 
is_________ .
Definitions:
Percentile
A measure indicating the value below which a given percentage of observations in a group of observations fall.
Quartile
A statistical term describing each of four equal groups into which a population can be divided based on a distribution.
Histogram
A graphical representation of the distribution of numerical data, showing the frequency of data points in consecutive, non-overlapping intervals.
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