Out of class question: A student has noticed significant differences between the

Question

Out of class question:

A student has noticed significant differences between the amount of suspended sediment in 2 tributaries of the Blackwood River (in the South-West of WA). The first tributary (Boyup Brook) is loaded with suspended sediment, whereas the second (Balingup Brook) is not. The student has a theory that this difference is due to a greater mean flow speed in Boyup Brook (the idea being that stronger flows create more erosion). The student goes to her office-mate who has been monitoring flow speeds in Boyup Brook over the past few years. The student takes her office-mate's data set and correctly reports the mean flow speed (and 95% confidence interval) as 32 + 9 cm/s. The standard deviation of the data set is 11.6 cm/s. (a) How many measurements of flow speed did her office-mate make in Boyup Brook?

(b) The student then realises that her office-mate has the same number of flow speed measurements in Balingup Brook. The mean flow speed (and 95% confidence interval) in Balingup Brook is 24 + 3 cm/s. What statement can she make comparing the flow speeds in the 2 tributaries?

(c) If you were designing an experiment, discuss two things that you would do to maximise your confidence in a difference between the mean flow speeds in these 2 tributaries.

Explanation / Answer

Mean flow speed= 32+9 cm/s

Standard deviation=11.6 cm/s

z score=1.96( Since 95% confidence interval)

N=sample size or no of obseravtion=?

ME=0.025

N=(z2-SD2)/ME2

=(1.96^2-11.6^2)/0.025^2

=(3.8416-134.56)/6.25*10e-4

=130.71/0.000625

since 0.000625 can be neglected

=130

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