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Sklearn KNeighborsClassifier (in Python) - K-Nearest Neighbors in Scikit-learn
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Nov 19, 2024
Original Tutorial: https://www.jcchouinard.com/k-nearest-neighbors/ ----- Install Python: - MacOS: https://www.jcchouinard.com/install-python-on-macos/ - Windows: https://www.jcchouinard.com/install-python-on-windows/ - Anaconda: https://www.jcchouinard.com/install-python-with-anaconda-on-windows/ Subscribe to my Python Course waitlist: https://ba995fe7.sibforms.com/serve/MUIEAPv50TFkNuoknCNvzRUKLLhvvZd5jzCwEZvi9BNjtkhVuEtLpEG58-Khdquyg9v5V-1qeGDwIXy739I4eFIVcCqeqqV3UUisW_-hAd5ljC1dGMrQvXHC7JvORh9TbnLA1CHqWro4N7YVZ4730-D5dXGxqd3CbaVHSJpS5fyylPMVzCe1_y9xOTl2-CsvEuhO01E0Ytv59HEJ Support my work: https://www.buymeacoffee.com/jcchouinard Subscribe to this channel: https://www.youtube.com/channel/UCthJJqyUGdI5EA0YjnjXthw?sub_confirmation=1 Follow me: https://twitter.com/ChouinardJC https://www.linkedin.com/in/jeanchristophechouinard/ https://www.buymeacoffee.com/jcchouinard
View Video Transcript
0:00
hi everyone my name is Ja in this Python
0:04
tutorial I'm going to show you how to
0:05
use psychic learns caners classifiers
0:08
and how to perform K rest neighbor in py
0:10
it learns so what KNN is uh it's an
0:15
algorithm used in supervised learning
0:18
for both classification and regression
0:20
and essentially is you try to classify
0:24
data in two or multiple categories based
0:27
on the closest neighbors so if if you
0:30
have some prediction here uh you can say
0:32
okay if I'm looking at three uh variable
0:36
uh in that Circle I'm going to have this
0:39
classification and so if K is equal
0:42
three then I will classify it as green
0:46
or Class B and if K is six since it's uh
0:50
bigger I have four items in that class
0:52
six and two in green then I'm
0:54
classifying it as Class A so in a
0:58
nutshell that's how K&N work
1:01
Works uh if you want to learn more uh
1:03
make sure that you uh look at my
1:06
tutorial
1:07
online uh otherwise let's get started so
1:11
I have loaded this data for you and
1:13
essentially what I'm doing is I'm
1:15
loading the breast cancer data set and I
1:18
have all these features that try to
1:21
predict something which is malignant or9
1:24
so I have these
1:26
features and what I'm trying to predict
1:29
is whether
1:30
cancer is malignant or benign in breast
1:34
cancer data
1:35
set uh so I'm going to since psychic
1:39
learn requires us to use uh numer
1:41
numerical data arrays I'm going to uh
1:44
assign the data set to X and Y and then
1:48
I'm going to get started with data
1:51
prepar preparation and splitting so
1:54
whenever we run machine learning model
1:58
what we want to do is we want to make
1:59
sure that we have a a test set and a
2:03
control set so that we can look at
2:05
accuracy and this is essentially what
2:07
this does you can look at my tutorial on
2:09
train test split if you want to
2:11
understand what this does but
2:13
essentially it creates uh it split my
2:16
data in a 30% test group so let's run
2:21
this and let's dive into the meat of the
2:25
subject which is the actual keris
2:28
neighbor and in in order to run a k KNN
2:32
algorithm with cych learn we need to uh
2:36
from SK learn the
2:39
neighbors so it's within neighbors
2:42
module you import K nearest neighbor K
2:47
neighbors
2:49
classifier and in order to train the
2:51
model we generally use the uh naming
2:54
convention KNN and we use K neighbors
2:57
classifier and the first parameter that
3:00
we want to use the parameter that we
3:02
want to set is the number of neighbors
3:04
and that's the K value I was talking to
3:06
you about so in this case we'll randomly
3:09
select eight and I'm going to show you
3:12
how to select that value later and then
3:15
we do KN ann. fit at X train and Y train
3:21
and then we will
3:26
predict using KNN
3:30
predict X test so here the fit method is
3:35
the actual training of the of the data
3:37
set so that we uh that's why we put the
3:39
training data in there and the predict
3:42
method is where we put in the test so
3:45
we're trying to predict y test uh so
3:48
here that's why we have splitted X test
3:52
y train and we're going to try to
3:54
predict y test and in order to uh
3:57
compare the score we're going to predict
3:59
compare y test to the actual y
4:02
prediction uh data and that's how uh we
4:07
compute accuracy so we do KNN doore and
4:11
we add X test and Y test so we will look
4:18
at that information and currently our
4:21
prediction accuracy is
4:24
92.9% so 92.9% of the time we are right
4:30
and that happens because this data set
4:32
uh is a clean data set so we don't have
4:34
too much pre-processing to do but let's
4:37
dive into that
4:41
information so what we want to know is
4:44
how many neighbors should we actually
4:46
put in the data so what I'm going to do
4:49
is I'm going to PL make a plot that will
4:51
tell us this information so I have the
4:54
same split and training the same data
4:57
here and I'm going to create variable
4:59
where to store the information so we
5:02
start with the same starting point of as
5:04
what we have done but what we're going
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to do is we're going to try to Loop
5:08
through uh multiple value values so what
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we can do is we can say
5:14
neighbors equal
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NP a range 1 to 26 so essentially we
5:23
what this NPA range will do is that it
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will create a range of value from 1 2 3
5:29
4 up to 26 and we are going to Loop for
5:33
each of these value so in this case if I
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print
5:38
neighbor you'll see that what I'm doing
5:40
is I'm actually looping through all the
5:43
value 1 to 25 and what we want to do is
5:47
we want
5:48
to train a model using that value every
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time so we use K neighbor classifier and
5:58
neighbors equal W and then we put the
6:01
number of the neighbor so here we will
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train it at one train it two and then
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we'll expand up to 25 in order to try to
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find the best um the best uh value for
6:15
en
6:16
neighbors so we will train the model at
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fit and X train y
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train and then what we will do is with
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the training value we will try
6:30
[Music]
6:32
to assign the test accuracy and the
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train accuracy the training accuracy in
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the test secur and here for each
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neighbor essentially we will
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assign the can and score so Cann do
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score so that's our accuracy score X
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train y train and then here I'm going to
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do the same but instead of X Trin y
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train it will be X
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test y
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test so I'm going to plot H so I'm going
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to have this um this value here and what
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I want to do is actually plot this
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information so I'm going to plot My
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Title Here with mad plot liim hence why
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uh I've imported mad plot
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liim uh and then I what I do is I will
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need to plot plot The Neighbors so I'm
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taking all the neighbors for the x value
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and I'm going to put the train
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accuracies
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values and the label here will
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be train
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accuracy and I can do the same with the
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uh test accuracies and say test accuracy
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and then I will plot a legend plot X
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label and plot show let me show you what
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this is going to do it just plots this
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information in order to tell us that
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somewhere around here before it plate uh
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we have the best possible and neighbors
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and over here to the right that's me
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that's where we start overfitting the
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data um so this is a plot that shows you
8:26
the accuracy by NE number of neighbor we
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can also actually do grid search CV in
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order to predict the right uh get the
8:36
the actual right um end neighbor so what
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we're going to do is we are going to run
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parent grid and I'm going to not going
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to go too deep into the grid search CV
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because this is a K&N
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uh uh K&N tutorial but let me show you
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here uh I'm going to use en neighor from
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NPA range from 1 to 50 uh and I'm going
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to use grid search CV KNN
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CV equal
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grid
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[Music]
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search
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CV and I'm going to use
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KNN param grid and CV equal five cross
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validation equal five let's not dive too
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too deep into what this means but we can
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use now KNN CV to fit on the xra and
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white train and what this will do is
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that it will go through all the end
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neighbor and fit the model and what it
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will show is we can show the print
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K&N
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CV best
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pars and we can
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print we can print the best score as
9:55
well so essentially by doing this we
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will be able to know what are the best
10:01
parameter and what will be the accuracy
10:03
score for this best series of of
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parameter in this case it tells us that
10:10
uh we have we should use six neighbor
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with an accuracy of almost
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95% and here when we show you the plot
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earlier we were around six parameters so
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we were rightous by looking at the plot
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but uh grid search CV it's a much better
10:26
way if we look at our PR previous we had
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90% uh 92% accuracy and now we are
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showing 95% accuracy which is
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fantastic now we can evaluate the model
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and one of the way that we can do this
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is by using confusion Matrix if you
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don't know what a confusion Matrix is I
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have a tutorial on the topic uh here on
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my website but essentially it shows you
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the true positive true negative false
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negatives and false posit postives um so
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essentially is how often do you how
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often you are right and how often you
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are wrong
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or uh I won't dive too much into this
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but you can uh look at that accuracy by
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using cm equal confusion
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Matrix and this is useful it's again not
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a torial on KNN but uh not on confuser
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Matrix but it's helpful to go when you
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look at your KNN uh accuracy you can
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look at your labels and then you do
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KNN
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classes so we are going to show this I'm
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putting some
11:44
color and I'm putting a display and I'm
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going to use confusion Matrix display
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because if I just run confusion Matrix
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what I have is an array like this which
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you can interpret if you want to make it
11:59
more visual then you come in to with
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confusion Matrix display and you can
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put display
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labels equal K&N classes so essentially
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you are just making a beautiful plot
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with
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this and then you can plot the title and
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plot the confusion Matrix plot and then
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you can see uh your true labels and if
12:34
you want to interpret this what it means
12:36
is that you have 102 instances where you
12:39
correctly predicted the
12:42
B9 uh and 57 instances when you
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correctly predicted
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malignant um let's not dive too much but
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you can interpret this and the way you
12:55
are going to go further is by looking at
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the classification report and that's
13:00
where you can have your metri actual
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metric so you go from escalar
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metric import classification
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report and then you can print
13:17
classification
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Report with
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your white test y bre so you're
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comparing your prediction against your
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initial test information and you get
13:34
that report again I have a tutorial here
13:37
that can uh tell you what the
13:39
classification report is uh but
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essentially what you want to learn is
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your uh accuracy here is
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93% um and that says how often you are
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right when
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predicting uh and the other information
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that you can look is here the class one
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so essentially if we come back to our
14:01
problem here we try to uh Define how
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often we can predict
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cancer um as malignant and so what we
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can look here we can look at the
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Precision and recall and what the
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Precision says is that when you
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predicted
14:23
cancer how often was it really cancer
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and the answer is 94% of the time when
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we predicted someone had cancer it was
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really cancer and the recall here is
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what percentage of the real cancers of
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all the cancers that existed so the true
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positive and the the the false negative
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uh how of of all the real cancers how
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what is the percentage that we could
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predict is 95% so these number in a
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cases of cancer they should be really
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really high and that's what we have in
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this
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case now let's dive into feature
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scaling uh here we have a specific case
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where the data set is quite clean but
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with KNN it's very uh KNN is really
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sensitive to the scale of the data so if
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you have uh something that are on a very
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large scale and some other features that
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are in a tiny scale this really can skew
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uh the calculations because the distance
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won't map the same thing here if you
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want to scale uh the data so you one way
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you can do this is you might want to
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convert uh the mean to be zero and the
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uh standard deviation to be
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one
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um and so essentially you have one
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standard deviation everywhere and one
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way you can do this is you can use
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standard scaler and and to do this you
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just instantiate a
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scaler and then you just run xtrain
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scaled equals
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scaler fit
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[Music]
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transform and you can run X test scale
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and you run scaler and then you just uh
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transform because you don't want to fit
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on on the test
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data forgot to say
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xend X test so you don't want to train
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on the test data that's why you're just
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transforming and if we plot the results
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here we have the original uh mean and
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and standard
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deviation and once we have transferred
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uh transformed it we have this mean and
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uh standard deviation
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in order to do this within the P
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Pipeline with uh uh can neighbors
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classif fires uh we start with the same
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initial step that we've done so far we
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run the train test plate and then we
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create a steps variable and the steps
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variable should be a list and the first
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the each step should be what you're
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trying to do here we have a scaler and
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we can run standard scal
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the list should be two
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polls and we have
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KNN and what we have here is K neighbors
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classifiers and neighbors should be six
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because we learned that information
17:49
earlier and then we train the data in a
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separate Way by running a pipeline so we
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instantiate pipeline equals pip pip
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line and within the pipeline we give the
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steps uh two pole then we can train the
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model using
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KNN scale equals pipeline instead of
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using KN andn the fit this time we use
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pipeline the FIP and we put in the X
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train y train and then we predict
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using the KNN
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scale predict on X test again and then
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we can print the pipeline score and
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that's how you train a model using a
18:44
pipeline so thank you very much that was
18:47
all for this K neighbors classifier with
18:49
pyit learn tutorial uh make sure to
18:52
subscribe to my Channel or visit my
18:54
website or follow me on the social media
18:57
thank you very much see you next next
18:59
time
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