Mean/ standard deviation

[whitehorsey]

1. An airline's flights have a mean arrival time of 5.8 min late with standard deviation of 2.1 min. What would be the appropriate x - scale for a histogram?



2. http://www.mathsrevision.net/gcse/sdeviation2.gif [Broken]



3. I tried to do this problem, but I couldn't think of anyway to find the x - scale. Can you please explain to me how? Thank You!

 

[CompuChip]

I don't know if this is really what they are asking, but in a normal distribution probability of finding a value outside two standard deviations (on either side) of the mean is about 5%. So usually, if you fit 3 ~ 5 standard deviations on your x-scale you are good.

 

[whitehorsey]

I don't know if this is really what they are asking, but in a normal distribution probability of finding a value outside two standard deviations (on either side) of the mean is about 5%. So usually, if you fit 3 ~ 5 standard deviations on your x-scale you are good.

There asking for what you said, but I don't get what u mean about the 5% and how did u get 3 ~ 5 standard deviations.

 

[CompuChip]

It's a rule of thumb for the normal distribution. See also the plot at http://en.wikipedia.org/wiki/Normal_distribution#Standard_deviation_and_confidence_intervals. As you can see, outside 3$\sigma$ the graph lies almost on the x-axis, so if you plot the values until about 3 (to be on the safe side, you can use 4 or 5) standard deviations on each side of the mean, you will get all relevant info in your graph.