lars/FYS-STK4155
1
0
Fork 0
Code Issues Pull Requests Actions Packages Projects Releases Wiki Activity

update book chapters

Browse Source
This commit is contained in:
Morten Hjorth-Jensen
2021-10-26 22:58:15 +02:00
parent 6c32c5b5e2
commit 5ee2934f9a
33 changed files with 367 additions and 202 deletions
Download Patch File Download Diff File Expand all files Collapse all files
doc/BookChapters/chapter3.do.txt
+29 -11
View File
@@ -1202,7 +1202,7 @@ standard deviation. Most machine learning libraries do this as a default. This m
the results may differ.
The
"Standadscaler":"https://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.StandardScaler.html"
"Standardscaler":"https://scikit-learn.org/stable/modules/generated/sklearn.preprocessing.StandardScaler.html"
function in _Scikit-Learn_ does this for us. For the data sets we
have been studying in our various examples, the data are in many cases
already scaled and there is no need to scale them. You as a user of different machine learning algorithms, should always perform a
@@ -1256,12 +1256,16 @@ The cost/loss function for regression is
C(\beta_0, \beta_1, ... , \beta_{p-1}) = \frac{1}{n}\sum_{i=0}^{n} \left(y_i - \beta_0 - \sum_{j=1}^{p-1} X_{ij}\beta_j\right)^2,.
\]
!et
Recall also that we use the squared value since this leads to an increase of the penalty for higher differences between predicted and output/target values.
What we have done is to single out the $\beta_0$ term in the definition of the mean squared error (MSE).
The design matrix
$X$ does in this case not contain any intercept column.
When we take the derivative with respect to $\beta_0$, we want the derivative to obey
Recall also that we use the squared value. This expression can lead to an
increased penalty for higher differences between predicted and
output/target values.
What we have done is to single out the $\beta_0$ term in the
definition of the mean squared error (MSE). The design matrix $X$
does in this case not contain any intercept column. When we take the
derivative with respect to $\beta_0$, we want the derivative to obey
!bt
\[
\frac{\partial C}{\partial \beta_j} = 0,
@@ -1282,7 +1286,7 @@ Multiplying away the constant $2/n$, we obtain
\]
!et
Let us special first to the case where we have only two parameters $\beta_0$ and $\beta_1$.
Let us specialize first to the case where we have only two parameters $\beta_0$ and $\beta_1$.
Our result for $\beta_0$ simplifies then to
!bt
\[
@@ -1298,10 +1302,10 @@ We obtain then
If we define
!bt
\[
\mu_1=\frac{1}{n}\sum_{i=0}^{n-1} (X_{i1},
\mu_{\bm{x}_1}=\frac{1}{n}\sum_{i=0}^{n-1} X_{i1},
\]
!et
and if we define the mean value of the outputs as
and the mean value of the outputs as
!bt
\[
\mu_y=\frac{1}{n}\sum_{i=0}^{n-1}y_i,
@@ -1310,16 +1314,30 @@ and if we define the mean value of the outputs as
we have
!bt
\[
\beta_0 = \mu_y - \beta_1\mu_{1}.
\beta_0 = \mu_y - \beta_1\mu_{\bm{x}_1}.
\]
!et
In the general case withmore parameters than $\beta_0$ and $\beta_1$, we have
In the general case with more parameters than $\beta_0$ and $\beta_1$, we have
!bt
\[
\beta_0 = \frac{1}{n}\sum_{i=0}^{n-1}y_i - \frac{1}{n}\sum_{i=0}^{n-1}\sum_{j=1}^{p-1} X_{ij}\beta_j.
\]
!et
We can rewrite the latter equation as
!bt
\[
\beta_0 = \frac{1}{n}\sum_{i=0}^{n-1}y_i - \sum_{j=1}^{p-1} \mu_{\bm{x}_j}\beta_j,
\]
!et
where we have defined
!bt
\[
\mu_{\bm{x}_j}=\frac{1}{n}\sum_{i=0}^{n-1} X_{ij},
\]
!et
the mean value for all elements of the column vector $\bm{x}_j$.
Replacing $y_i$ with $y_i - y_i - \overline{\bm{y}}$ and centering also our design matrix results in a cost function (in vector-matrix disguise)