The range that k must be to make this an effective screening device is "33000<=K<=45000".
In statistic, range of a data, refers to difference between the largest and smallest values. Range is often the result of subtracting the sample maximum and minimum which is expressed in the same units as the data.
It is also size of the smallest interval which contains all the data and provides an indication of statistical dispersion. As its depends on two of observations, it is most useful in representing the dispersion of small data sets.
High-ability range:
= 8000 + 25000
= 33000
Low-ability range:
= 20000 + 25000
= 45000.
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