If σ is unknown and there is no preliminary sample available to estimate σ, the range divided by four can be utilized to determine an estimate of σ when finding sample size.

Name: If σ is unknown and there is no preliminary sample available to estimate σ, the range divided by four can be utilized to determine an estimate of σ when finding sample size.
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Quizplus · Accepted Answer

This gives a conservatively large estimate of the standard deviation.

If σ Is Unknown and There Is No Preliminary Sample