Fys-Stk4155 Project 1 Solved

Regression analysis and resampling methods

The main aim of this project is to study in more detail various regression methods, including the Ordinary Least Squares (OLS) method, The total score is 100 points. Each subtask has its own final score.

We will first study how to fit polynomials to a specific two-dimensional function called Franke’s function. This is a function which has been widely used when testing various interpolation and fitting algorithms. Furthermore, after having established the model and the method, we will employ resamling techniques such as cross-validation and/or bootstrap in order to perform a proper assessment of our models. We will also study in detail the so-called Bias-Variance trade off.

The Franke function, which is a weighted sum of four exponentials reads as follows

3 􏰄 f(x,y)=4exp −

1 􏰄 +2exp −

(9x − 2)2 4 (9x − 7)2

4

(9y − 2)2 􏰅 3 􏰄

(9x + 1)2 49

(9y + 1) 􏰅 10

+4exp − (9y − 3)2 􏰅 1 􏰀

− 4 − 4

−
−5exp −(9x−4) −(9y−7) .

2

2􏰁

The function will be defined for x, y ∈ [0, 1]. Our first step will be to perform an OLS regression analysis of this function, trying out a polynomial fit with an x andydependenceoftheform[x,y,x2,y2,xy,…]. Wewillalsoincludebootstrap first as a resampling technique. After that we will include the cross-validation technique. As in homeworks 1 and 2, we can use a uniform distribution to set up the arrays of values for x and y, or as in the example below just a set of fixed values for x and y with a given step size. We will fit a function (for example a polynomial) of x and y. Thereafter we will repeat much of the same procedure using the Ridge and Lasso regression methods, introducing thus a dependence on the bias (penalty) λ.

Finally we are going to use (real) digital terrain data and try to reproduce these data using the same methods. We will also try to go beyond the second- order polynomials metioned above and explore which polynomial fits the data best.

The Python code for the Franke function is included here (it performs also a three-dimensional plot of it)

from mpl_toolkits.mplot3d import Axes3D
import matplotlib.pyplot as plt
from matplotlib import cm
from matplotlib.ticker import LinearLocator, FormatStrFormatter
import numpy as np

from random import random, seed

fig = plt.figure()
ax = fig.gca(projection=’3d’)

# Make data.
x = np.arange(0, 1, 0.05)
y = np.arange(0, 1, 0.05)
x, y = np.meshgrid(x,y)

def FrankeFunction(x,y):
    term1 = 0.75*np.exp(-(0.25*(9*x-2)**2) - 0.25*((9*y-2)**2))
    term2 = 0.75*np.exp(-((9*x+1)**2)/49.0 - 0.1*(9*y+1))
    term3 = 0.5*np.exp(-(9*x-7)**2/4.0 - 0.25*((9*y-3)**2))
    term4 = -0.2*np.exp(-(9*x-4)**2 - (9*y-7)**2)
    return term1 + term2 + term3 + term4

z = FrankeFunction(x, y)

# Plot the surface.
surf = ax.plot_surface(x, y, z, cmap=cm.coolwarm,

                       linewidth=0, antialiased=False)

# Customize the z axis.
ax.set_zlim(-0.10, 1.40)
ax.zaxis.set_major_locator(LinearLocator(10))
ax.zaxis.set_major_formatter(FormatStrFormatter(’%.02f’))

# Add a color bar which maps values to colors.
fig.colorbar(surf, shrink=0.5, aspect=5)

plt.show()

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Exercise 1: Ordinary Least Square (OLS) on the Franke function (score 10 points). We will generate our own dataset for a function FrankeFunction(x, y) with x, y ∈ [0, 1]. The function f (x, y) is the Franke function. You should explore also the addition of an added stochastic noise to this function using the normal distribution N(0,1).

Write your own code (using either a matrix inversion or a singular value decomposition from e.g., numpy ) or use your code from homeworks 1 and 2 and perform a standard least square regression analysis using polynomials in x and y up to fifth order.

Evaluate the Mean Squared error (MSE) 1 n−1

MSE(y,y ̃)= 􏰇(yi−y ̃i)2,

n

i=0
and the R2 score function. If y ̃i is the predicted value of the i − th sample

and yi is the corresponding true value, then the score R2 is defined as

􏰆n−1(yi − y ̃i)2 2 i=0

R ( y , y ̃ ) = 1 − 􏰆 n − 1 ( y i − y ̄ ) 2 , i=0

where we have defined the mean value of y as 1 n−1

y ̄= 􏰇yi.

i=0

Plot the resulting scores (MSE and R2) as functions of the polynomial degree (here up to polymial degree five). Plot also the parameters β as you increase the

order of the polynomial. Comment your results.
Your code has to include a scaling/centering of the data (for example by

subtracting the mean value), and a split of the data in training and test data. For this exercise you can either write your own code or use for example the function for splitting training data provided by the library Scikit-Learn (make sure you have installed it). This function is called train_test_split. You should present a critical discussion of why and how you have scaled or not scaled the data.

It is normal in essentially all Machine Learning studies to split the data in a training set and a test set (eventually also an additional validation set). There is no explicit recipe for how much data should be included as training data and say test data. An accepted rule of thumb is to use approximately 2/3 to 4/5 of the data as training data.

You can easily reuse the solutions to your exercises from week 35 and week 36.

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n

Exercise 2: Bias-variance trade-off and resampling techniques (score 15 points). Our aim here is to study the bias-variance trade-off by implementing the bootstrap resampling technique.

With a code which does OLS and includes resampling techniques, we will now discuss the bias-variance trade-off in the context of continuous predictions such as regression. However, many of the intuitions and ideas discussed here also carry over to classification tasks and basically all Machine Learning algorithms.

Before you perform an analysis of the bias-variance trade-off on your test data, make first a figure similar to Fig. 2.11 of Hastie, Tibshirani, and Friedman. Figure 2.11 of this reference displays only the test and training MSEs. The test MSE can be used to indicate possible regions of low/high bias and variance. You will most likely not get an equally smooth curve!

With this result we move on to the bias-variance trade-off analysis. ConsideradatasetLconsistingofthedataXL ={(yj,xj),j=0…n−1}. Let us assume that the true data is generated from a noisy model

y = f(x) + ε.

Here ε is normally distributed with mean zero and standard deviation σ2.

In our derivation of the ordinary least squares method we defined then an approximation to the function f in terms of the parameters β and the design matrix X which embody our model, that is y ̃ = Xβ.

The parameters β are in turn found by optimizing the means squared error via the so-called cost function

1 n−1

C(X,β)= 􏰇(yi−y ̃i)2=E􏰂(y−y ̃)2􏰃.

n

i=0
Here the expected value E is the sample value.

Show that you can rewrite this as
E 􏰂(y − y ̃)2􏰃 = n1 􏰇(fi − E [y ̃])2 + n1 􏰇(y ̃i − E [y ̃])2 + σ2.

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Explain what the terms mean, which one is the bias and which one is the variance and discuss their interpretations.

Perform then a bias-variance analysis of the Franke function by studying the MSE value as function of the complexity of your model.

Discuss the bias and variance trade-off as function of your model complexity (the degree of the polynomial) and the number of data points, and possibly also your training and test data using the bootstrap resampling method.

Note also that when you calculate the bias, in all applications you don’t know the function values fi. You would hence replace them with the actual data points yi.

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Exercise 3: Cross-validation as resampling techniques, adding more complexity (score 15 points). The aim here is to write your own code for another widely popular resampling technique, the so-called cross-validation method. Again, before you start with cross-validation approach, you should scale your data if you think this is needed.

Implement the k-fold cross-validation algorithm (write your own code) and evaluate again the MSE function resulting from the test folds. You can compare your own code with that from Scikit-Learn if needed.

Compare the MSE you get from your cross-validation code with the one you got from your bootstrap code. Comment your results. Try 5 − 10 folds. You can also compare your own cross-validation code with the one provided by Scikit-Learn.

Exercise 4: Ridge Regression on the Franke function with resampling (score 20 points). Write your own code for the Ridge method, either using matrix inversion or the singular value decomposition as done in the previous exercise. Perform the same bootstrap analysis as in the Exercise 2 (for the same polynomials) and the cross-validation in exercise 3 but now for different values

of λ. Compare and analyze your results with those obtained in exercises 1-3. Study the dependence on λ.

Study also the bias-variance trade-off as function of various values of the parameter λ. For the bias-variance trade-off, use the bootstrap resampling method. Comment your results.

Exercise 5: Lasso Regression on the Franke function with resampling (Score 10 points)). This exercise is essentially a repeat of the previous two ones, but now with Lasso regression. Write either your own code (difficult and optional) or, in this case, you can also use the functionalities of Scikit-Learn (recommended). Give a critical discussion of the three methods and a judgement

of which model fits the data best. Perform here as well an analysis of the bias- variance trade-off using the bootstrap resampling technique and an analysis of the mean squared error using cross-validation.

Exercise 6: Analysis of real data (score 30 points). With our codes functioning and having been tested properly on a simpler function we are now ready to look at real data. We will essentially repeat in this exercise what was done in exercises 1-5. However, we need first to download the data and prepare properly the inputs to our codes. We are going to download digital terrain data from the website https://earthexplorer.usgs.gov/,

Or, if you prefer, we have placed selected datafiles at https://github.com/ CompPhysics/MachineLearning/tree/master/doc/Projects/2021/Project1/ DataFiles

In order to obtain data for a specific region, you need to register as a user (free) at this website and then decide upon which area you want to fetch the digital terrain data from. In order to be able to read the data properly, you

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need to specify that the format should be SRTM Arc-Second Global and download the data as a GeoTIF file. The files are then stored in tif format which can be imported into a Python program using

scipy.misc.imread

Here is a simple part of a Python code which reads and plots the data from such files

import numpy as np
from imageio import imread
import matplotlib.pyplot as plt
from mpl_toolkits.mplot3d import Axes3D
from matplotlib import cm

# Load the terrain
terrain1 = imread(’SRTM_data_Norway_1.tif’)
# Show the terrain
plt.figure()
plt.title(’Terrain over Norway 1’)
plt.imshow(terrain1, cmap=’gray’)
plt.xlabel(’X’)
plt.ylabel(’Y’)
plt.show()

If you should have problems in downloading the digital terrain data, we provide two examples under the data folder of project 1. One is from a region close to Stavanger in Norway and the other Møsvatn Austfjell, again in Norway. Feel free to produce your own terrain data.

Alternatively, if you would like to use another data set, feel free to do so. This could be data close to your reseach area or simply a data set you found interesting. See for example kaggle.com for examples.

Our final part deals with the parameterization of your digital terrain data (or your own data). We will apply all three methods for linear regression, the same type (or higher order) of polynomial approximation and cross-validation

as resampling technique to evaluate which model fits the data best.
At the end, you should present a critical evaluation of your results and discuss the applicability of these regression methods to the type of data presented here

(either the terrain data we propose or other data sets).

Background literature

1. For a discussion and derivation of the variances and mean squared errors using linear regression, see the Lecture notes on ridge regression by Wessel N. van Wieringen

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2. The textbook of Trevor Hastie, Robert Tibshirani, Jerome H. Friedman, The Elements of Statistical Learning, Springer, chapters 3 and 7 are the most relevant ones for the analysis here.

Introduction to numerical projects

Here follows a brief recipe and recommendation on how to answer the various questions when preparing your answers. Note that you can answer question by question and there is no need to structure your report as a scientific report with abstract, introduction, theory, results and discussions, conclusions etc. But you have the following elements in mind when you answer the various questions.

• Give a short description of the nature of the problem and the eventual numerical methods you have used.
• Describe the algorithm you have used and/or developed. Here you may find it convenient to use pseudocoding. In many cases you can describe the algorithm in the program itself.
• Include the source code of your program. Comment your program properly. You should have the code at your GitHub/GitLab link. You can also place the code in an appendix of your report.
• If possible, try to find analytic solutions, or known limits in order to test your program when developing the code.
• Include your results either in figure form or in a table. Remember to label your results. All tables and figures should have relevant captions and labels on the axes.
• Try to evaluate the reliabilty and numerical stability/precision of your results. If possible, include a qualitative and/or quantitative discussion of the numerical stability, eventual loss of precision etc.
• Try to give an interpretation of you results in your answers to the problems.
• Critique: if possible include your comments and reflections about the exercise, whether you felt you learnt something, ideas for improvements and other thoughts you’ve made when solving the exercise. We wish to keep this course at the interactive level and your comments can help us improve it.
• Try to establish a practice where you log your work at the computerlab. You may find such a logbook very handy at later stages in your work, especially when you don’t properly remember what a previous test version of your program did. Here you could also record the time spent on solving the exercise, various algorithms you may have tested or other topics which you feel worthy of mentioning.

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Format for electronic delivery of report and programs

The preferred format for the report is a PDF file. You can also use DOC or postscript formats or as an ipython notebook file. As programming language we prefer that you choose between C/C++, Fortran2008, Julia or Python. The following prescription should be followed when preparing the report:

• Use Canvas to hand in your projects, log in at https://www.uio.no/ english/services/it/education/canvas/ with your normal UiO user- name and password.
• Upload only the report file or the link to your GitHub/GitLab or similar typo of repos! For the source code file(s) you have developed please provide us with your link to your GitHub/GitLab or similar domain. The report file should include all of your discussions and a list of the codes you have developed. Do not include library files which are available at the course homepage, unless you have made specific changes to them.
• In your GitHub/GitLab or similar repository, please include a folder which contains selected results. These can be in the form of output from your code for a selected set of runs and input parameters.

  Finally, we encourage you to collaborate. Optimal working groups consist of 2-3 students. You can then hand in a common report.

  Software and needed installations

  If you have Python installed (we recommend Python3) and you feel pretty familiar with installing different packages, we recommend that you install the following Python packages via pip as

  1. pip install numpy scipy matplotlib ipython scikit-learn tensorflow sympy pandas pillow

  For Python3, replace pip with pip3.
  See below for a discussion of tensorflow and scikit-learn.
  For OSX users we recommend also, after having installed Xcode, to install

  brew. Brew allows for a seamless installation of additional software via for example

  1. brew install python3

  For Linux users, with its variety of distributions like for example the widely popular Ubuntu distribution you can use pip as well and simply install Python as

  1. sudo apt-get install python3 (or python for python2.7)

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etc etc.
If you don’t want to install various Python packages with their dependencies

separately, we recommend two widely used distrubutions which set up all relevant dependencies for Python, namely

1. Anaconda Anaconda is an open source distribution of the Python and R programming languages for large-scale data processing, predictive analytics, and scientific computing, that aims to simplify package management and deployment. Package versions are managed by the package management system conda
2. Enthought canopy is a Python distribution for scientific and analytic computing distribution and analysis environment, available for free and under a commercial license.

Popular software packages written in Python for ML are • Scikit-learn,

• Tensorflow, • PyTorch and • Keras.

These are all freely available at their respective GitHub sites. They encompass communities of developers in the thousands or more. And the number of code developers and contributors keeps increasing.

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