Applying Excel to Statistics

[Lundy96]

To whomever is reading this, I have been given a series of questions by my boss in order to try taking statistical problems and using excel functions in order to derive the answers. I am completely new to both the fields of excel and statistics and would like some guidance on how to go about these:

a) The time taken for field workers to mark out a plot for logging is normally distributed with a mean of 75 minutes and a standard deviation of 15 minutes. What is the probability that the time is between 70 and 90 minutes? (You need to use an appropriate Excel function to determine the probability.)

b) A logger knows that, on average, it takes two hours to clear eleven tree roots. Find the probability that a logger will clear at most five tree roots in a one hour period.

c) If the probability of finding a tree species with an extensive subterranean root system is 0.43 and five plots were selected, find the probability that at most two of the six plots contain a tree species with an extensive subterranean root system.

d) Can we conclude at the 10% significance level that logging significantly reduces the number of species in a plot after a decade? Here we want you to highlight the relevant critical value or p-value and the test statistic to aid you in interpreting the data.

I know these may seem basic to some but I'm only just starting to dip my feet in this (as I am an intern who is asked to learn) and would really appreciate some help or guidance on how to go about doing this. Thank you so much in advance!

 

[steve the fish]

Hi. Is this your homework from your boss???

 

[Lundy96]

I wish it was homework.... Its what he gave me in the morning and told me to spend the day completing. Said I can use "whatever means" to do it but must later come and explain to him the theory behind it....

First of all I am an arts student who is (obviously) doing an internship out of their comfort zone, and secondly my boss scares the crap out of me and I dont know how else to go about this Please help me Steve or anyone else who can!

 

[Lundy96]

So I worked on the problems a bit and have come up with the respective workings for each problem (will follow this sentence), can someone tell me if I am going in the right direction or what I am supposed to do differently:

Part A
Question: The time taken for field workers to mark out a plot for logging is normally distributed with a mean of 75 minutes and a standard deviation of 15 minutes. What is the probability that the time is between 70 and 90 minutes? (You need to use an appropriate Excel function to determine the probability.)

Answer: Through some research I found that the best way to go about doing this is by using the following normal distribution to calculate the probability and the formula i used is =NORM.DIST(90,75,15,TRUE)-NORM.DIST(70,75,15,TRUE). This gave me the probability of 47% (0.4719). Is this the correct way to go about answering this or am I doing it wrong?

Part B
Question: A logger knows that, on average, it takes two hours to clear eleven tree roots. Find the probability that a logger will clear at most five tree roots in a one hour period.

Answer: I followed a Poisson Distribution to calculate this using the equation =POISSON.DIST(5,5.5,TRUE) where I assume that since 11 is the average in 2 hours then 5.5 is the average in 1 hour, and from this I get a probability of 53% (0.5289). In this what I am wondering is that can mean be divided by two if the time is halved?

Part C
Question: the probability of finding a tree species with an extensive subterranean root system is 0.43 and five plots were selected, find the probability that at most two of the six plots contain a tree species with an extensive subterranean root system.

Answer: In this I used a Binomial Distribution with the equation BINOM.DIST(2,5,0.43,TRUE)=63% (0.6295) where I am telling excel that I wish to find out for 2 instances where previous experiment has had 0.43 probability from 5 plots. Here the confusion is the wording of the question where it says 2 out of 6 plots which makes me wonder if I am supposed to alter the data to be similar to the sample set of 5.