undetectable – an amount smaller than a test can measure. For viral load tests, this means below 400 or, more commonly, below 50 copies/mL, depending on the make of test. In research laboratories, some very sensitive tests can measure down to 1 copy/mL.

uneven distribution – a term in statistics where most results fall to the left (positive skewness) or right (negative skewness) of the middle of the range.