Comparing Two Means: Examples & Assumptions

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EvLer
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It's sort of a conceptual question:
one of the assumptions for 2-sample t confidence interval is equality of variances in two populations, and I need to come up with a concrete example of when this is likely to fail.
I sort of have an idea that it really depends how one chooses samples. For example, if one compares mean height of males and females on a certain campus where females are predominantly asian. Then since variance in height of asian women is much smaller than variance in height of white males, the assumption fails. ALthough I am not sure if this is a good example because I make an explicit assumption that asian females have small variance in height. Could someone help me out a bit with an example?
Thanks in advance.
 
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When what is likely to fail?
 
equality of the variances of the 2 populations
 
The probability of the two variances being equal is exactly zero.