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R代写 - ST346: Assessed coursework

时间：2020-11-20

Your solutions should be submitted electronically in the form of a PDF document using the submission
portal on the ST346 Moodle page. Please remember to include only your ID number on your submission
to allow anonymous marking.
If you have any queries about the coursework please post them on the ST346 forum, but do not post any
part of your solutions. This assignment counts towards 10% of your final module mark.
The maximum score for this coursework is 20/20. Numbers in brackets indicate the points available for
each question.
To access the data needed for this assignment, download the file courseworkData2.rda from the ST346
Moodle web page and read it into R using the function load(). This will create a copy of 2 data frames
in your R workspace: ships and esoph.
1. The data frame ships contains data on damage incidents in ships. The ships have been aggregated
into classes defined by type (5 categories), year of construction (4 categories), and period of
operation (2 categories), giving 5 × 4 × 2 = 40 rows. For each combination of type, year and
period, the total months of service of all ships in that class has been aggregated in the variable
service, and the total number of damage incidents has been aggregated in the variable incidents.
Note that some classes are empty. For these classes, the variable service is set to zero and the
variable incidents is set to the missing value NA. Rows with missing values are automatically
removed by R when you fit a statistical model.
(a) Fit an appropriate GLM to model the risk of damage incidents in terms of the other variables.
[2]
(b) Which combination of ship type, year of construction and period of operation is at highest risk
of damage? [2]
(c) Calculate the expected number of damage incidents in twelve months for this class of shipping.
[2]
2. Consider the following scenarios. Determine the appropriate model to use in each case. You should
give the exponential dispersion model (or family in R terminology), the link function, the outcome
variable, and the predictor variables.
(a) A cohort of 18 year-old school leavers were surveyed regarding their future plans, with specific
interest in higher education. Several variables were measured, including age, gender, smoking
status, as well as whether or not they had a place at university or other higher education institute.
We wish to investigate the variables which are associated with higher education attendance.[1]
(b) Sixty rats of the same age were divided into 2 groups of 30. At the start of the experiment,
all animals were weighed. Then one group was fed a control diet, whilst the other was fed a
diet supplemented with vitamin D. After 6 weeks, all rats were weighed again. We wish to
understand how vitamin D supplementation affects weight gain. [1]
(c) A car manufacturer conducted a study to investigate the reliability of their cars. They measured
the number of times that each of 1000 vehicles had broken down in the 10 years since it was
1
made. They also measured the number of miles that each car had been driven as well as the
model of each vehicle and the number of times that it had been serviced. [1]
3. The data set esoph concerns a study of esophageal cancer conducted in the Ille-et-Vilaine department
of France in the 1970s. Participants have been classified by age group (agegp), alcohol consumption
in g/day (alcgp) and tobacco consumption in g/day (tobgp). In each class defined by these variables,
ncases gives the number of participants in the study with esophageal cancer and ncontrols gives
the number of healthy controls without cancer.
(a) Fit an appropriate GLM to these data to model the risk of esophageal cancer. [2]
(b) The variables agegp, alcgp, and tobgp are factors. They can be converted to numeric variables
using the function as.numeric(). Determine whether each variable should be a factor or a
numeric variable using an appropriate methodology for model choice. [2]
(c) Using your final choice of model, estimate the odds ratio of esophageal cancer for a participant
who drinks more than 120g of alcohol/day compared with one who drinks less than 40g of
alcohol per day. [2]
4. Consider an experiment in which it is known that the variance of the errors of the first two observations
(i = 1, i = 2) is 3 times the variance of the errors for the next three observations (i = 3, i = 4, i = 5).
The errors are assumed to be uncorrelated. You are given the following model:
E (yi | β) = µi = β0 i = 1 or i = 4, β0 + β1 i = 2 or i = 5, β0 + 2β1 i = 3.
and you may assume that Var(yi | β) = σ2
Find the design matrix for this model and then calculate the maximum likelihood estimate of β1 to
2 decimal places when yT = c(115.0, 72.5, 22.5, 110.0, 60.0). You should do this calculation twice:
once by calling the lm() function in R and once by solving the weighted normal equations (or score
equations) for this model. [5]