In the video above, I did not mention an important point. When deciding whether to use R/4 or R/6, consider how large the sample that provided your maximum and minimum was. If it is a large sample, you're more likely to have some values that are three standard deviations away from average. In that case, use R/6. For smaller samples, those extreme values are less likely to turn up, so opt for R/4.

If you are using a thought experiment to approximate the standard deviation, you would use R/4 if you use values for the maximum and minimum that aren't too extreme. For example, if you choose values that you guess would bound between 80% - 90% of the population. It might be best to divide the range by 4 to approximate your standard deviation. If you choose values that are more extreme for your maximum and minimum, it is better to use R/6.

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Range Rule of Thumb

Sample Exams