# Question: We sometimes know the value of one population standard deviation

We sometimes know the value of one population standard deviation from an extensive history of data, but a new procedure or treatment results in sample values with an unknown standard deviation. If σ1 is known but σ2 is unknown, use the procedures in Part 1 of this section with these changes: Replace s1 with the known value of σ1 and find the number of degrees of freedom using the expression below. Repeat Exercise 13 by assuming that σ = 15 for proctored tests.

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