# Practical

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```					Practical

Thursday

Linear Modelling

1. In this exercise you will reproduce analyses in the lecture.
a. Read the data set “sim30.txt” into a data frame using read.table()
b. Print out the data (it should have 3 columns “x”, “xx”, “y” and 30 rows)
c. Plot x vs y and xx vs y
d. Compute the Pearson and Spearman correlation coefficients and test them using
cor.test()
e. Use the lm() function to fit a linear model y ~ x
f. Use the anova() function to test the significance of the fit.
g. Show that the F statistic in anova() is the square of the T statistic in cor.test()

2. In this exercise you will reproduce analyses in the lecture related to a dataset of Biochemistry
measurements on mice.
a. Read in the data set “Biochemistry.txt” into a data frame
b. Print out the first few lines using head() and find the dimensions of the complete data
set using dim()
c. Plot HDL against Tot.Cholesterol
d. Compute the Pearson and Spearman correlation coefficients and test them.
e. Fit a linear regression between HDL and Tot.Cholesterol using lm() and anova() and
compare the results to correlation.
f. One important way of visualising the fit of the model is to plot the original observations
against the predictions from the model. R will give you the necessary values; if f <- lm()
then f is a list containing the elements f\$fitted.values and f\$y. However, you must first
repeat the lm analysis using the additional option lm( formula, data, y=TRUE) .What
happens if you plot the fitted values against the original data?.
g. Perform a one-way analysis of variance on HDL against Family, using lm() and anova().
Plot the observed against the residuals.
h. Perform a non-Parametric analysis of the dependence between HDL and family.

3. In this exercise we explore the Biochemistry data set in more detail. All the columns with the
prefix “Biochem” are numerical measures of various chemicals in serum. The remaining columns
are covariates , ie variables collected at the same time that might influence the biochemistry
measures. One of these is Family, used in Ex 2. Another is GENDER, the sex of the animal.
a. Fit the HDL ~ GENDER, where GENDER is treated first as a numeric variable and second
as a factor. You can do this with the models Biochem.HDL ~ as.numeric(GENDER) and
Biochem.HDL ~ as.factor(GENDER) What do you notice about the anova() results of the
two models? Why is this?
b. Investigate which of the Biochem variables depend on GENDER, ie find out which
chemical signatures are different for males and females. You should try to write an R
function which will loop through the all variables with prefix Biochem, fit a linear model
and store the results of the anova in a table. To do this, you will need to know a fact
about the anova() function: If you store the results of anova in a variable, say a <-
anova() then a is a data frame and the P-value of the F test (in this example) is the
element a[1,6] . Use this fact to extract the P-value from each model fit and put it into a
data frame that you should create that also has the Biochem variable name as one of its
columns.
c. Now estimate the heritabilities of all the Biochem variables by adapting the function you
wrote in (b).
d. Modify your function to also compute the non-parametric analysis of Family effects.

4. Investigate the multivariate capabilities of the cor() function. You should first read the help on
this function by typing ?cor. You will also learn about a new plotting function image()
a. Find out how to compute all the pairwise correlations of the Biochem variables
simultaneously, storing the results in a matrix. When you try to compute it you must
decide how to treat missing data (cor() won’t compute a correlation if any data is
missing, by default).
b. Now investigate the plotting function image() to display the results. In order to put
labels on the figure, you need to know that this works:
i. image(mat,axes=FALSE); axis(

5. Optional Exercise for the mathematically minded (do this off line)
a. Show that the formulae given for least-squares estimators in the linear regression model
are correct, eg by differentiating the residual SS.
b. Show that the LS estimates partition the TSS into FSS and RSS as in the lecture
c. Show that the Pearson Correlation Coefficient is related to the ANOVA F-statistic.

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 views: 4 posted: 2/10/2012 language: English pages: 2