Question

 Define the following:
(i) Covariance Matrix
(ii) Mahalanobis D2
(iii) Hotelling’s T2
(iv) Clustering
(v) Relationship between (ii) and (iii). 

02 Apr 2024
Answer :
Word Count : 357

(i) Covariance Matrix:
A covariance matrix is a square matrix that summarizes the covariance between multiple variables. It shows how much two variables change together. In a covariance matrix, the element in the i-th row and j-th column represents the covariance between the i-th and j-th variables.

(ii) Mahalanobis D²:
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Related Questions

 Define the following:
(i) Covariance Matrix
(ii) Mahalanobis D2
(iii) Hotelling’s T2
(iv) Clustering
(v) Relationship between (ii) and (iii). 

Let X\sim N_4\left ( \mu ,\sum \right ), where \mu =\begin{pmatrix} 3\\ -2 \\1 \\ -2 \end{pmatrix}  and \sum=\begin{pmatrix} 4 & 0 & 0 & 0\\0 & 3 & 0 &0 \\ 0 & 0 & 2 & -2\\ 0 & 0 & -2 & 5 \end{pmatrix}. Check the independence of the (i) X_2 and X_1 (ii) (X_2 ,X_4  ) and ( X_1 , X_3 ) (iii) ( X_1 , X_2 ) and ( X_3 , X_4 ).

Let\underset{\sim }{X} =\binom{X_1}{X_2}  has the following joint density function

f\left ( X_1,X_2 \right )=\left\{\begin{matrix} 4x_1x_2 &,0<x_1<1,0<x_2<1, \\ 0 & ,\textup{otherwise.} \end{matrix}\right.

Find the marginal distributions, mean vector and variance-covariance matrix. Also, comment on the independence of X_1 and X_2

Obtain the maximum likelihood estimator of the mean vector and variance-covariance matrix of the multivariate normal distribution.

Let X be a 3-dimensional random vector with dispersion matrix  

\sum=\begin{pmatrix} 4 &-2 & 0\\ -2 & 4 &0 \\ 0 & 0 & 2 \end{pmatrix}.Determine the first principal component and the proportion of the total variability that it explains.

X\sim N_3\left ( \mu ,\sum \right ) with \mu=\begin{pmatrix} 2\\ 1 \\ 2 \end{pmatrix}  and \sum=\begin{pmatrix} 5 & 3&0 \\ 3 &3 & -2\\ 0 & -2 & 5 \end{pmatrix}.  Then find the joint distribution of   X_1+2X_2,2X_1-X_2 and X_3.

Consider the following data of 11 samples on 8 variables by Anscombe, Francis J. (1973):

x_1 x_2 x_3 x_4 y_1 y_2 y_3 y_4
10 10 10 8 8.04 9.14 7.46 6.58
8 8 8 8 6.95 8.14 6.77 5.76
13 13 13 8 7.58 8.74 12.74 7.71
9 9 9 8 8.81 8.77 7.11 8.84
11 11 11 8 8.33 9.26 7.81 8.47
14 14 14 8 9.96 8.10 8.84 7.04
6 6 6 8 7.24 6.13 6.08 5.25
4 4 4 19 4.26 3.10 5.39 12.50
12 12 12 8 10.84 9.13 8.15 5.56
7 7 7 8 4.82 7.26 6.42 7.91
5 5 5 8 5.68 4.74 5.73  

If the vector x=\begin{pmatrix} x_1\\ x_2 \\x_3 \\ x_4 \end{pmatrix}  and y=\begin{pmatrix} y_1\\y_2 \\ y_3 \\ y_4 \end{pmatrix}  then obtain the sample covariance matrix between x and y

Source: Anscombe, Francis J. (1973). Graphs in statistical analysis. The American Statistician, 27, 17– 21. doi: 10.2307/2682899.

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