The methods are in turn combined with resampling techniques like the bootstrap method and cross validation. These are discussed during weeks 36 and 37.
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
The function will be defined for. Our first step will be to perform an OLS regression analysis of this function, trying out a polynomial fit with an and dependence of the form . We will also include bootstrap 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 and , or as in the example below just a set of fixed values for and with a given step size. We will fit a function (for example a polynomial) of and . Thereafter we will repeat much of the same procedure using the Ridge and Lasso regression methods, introducing thus a dependence on the bias (penalty) .
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()
Part a): Ordinary Least Square (OLS) on the Franke function
We will generate our own dataset for a functionwith . The function is the Franke function. You should explore also the addition of an added stochastic noise to this function using the normal distribution .
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 confidence intervals of the parameters (estimators) by computing their variances, evaluate the Mean Squared error (MSE)and up to fifth order. Find the
and thescore function. If is the predicted value of the sample and is the corresponding true value, then the score is defined as
where we have defined the mean value ofas
Your code has to include a scaling of the data (for example by subtracting the mean value, see also homework set 2 for examples) and a split of the data in training and test data. For this part 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 . Similarly, and see the solution to homework set 2, exercise 2, you can use the data normalization functionality of Scikit-Learn.
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 approximatelyto of the data as training data.
Part b): Bias-variance trade-off and resamplng techniques
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.
Consider a datasetconsisting of the data .
Let us assume that the true data is generated from a noisy model
Hereis normally distributed with mean zero and standard deviation .
In our derivation of the ordinary least squares method we defined then an approximation to the functionin terms of the parameters and the design matrix which embody our model, that is .
The parametersare in turn found by optimizing the means squared error via the so-called cost function
Here the expected valueis the sample value.
Show that you can rewrite this as
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. You would hence replace them with the actual data points .
Part c) Cross-validation as resampling techniques, adding more complexity
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.
Implement the-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. Tryfolds. You can also compare your own cross-validation code with the one provided by Scikit-Learn.
Part d): Ridge Regression on the Franke function with resampling
Write your own code for the Ridge method, either using matrix inversion or the singular value decomposition as done in the previous exercise or howework 2 (see also chapter 3.4 of Hastie et al., equations (3.43) and (3.44)). Perform the same bootstrap analysis as in the part b) (for the same polynomials) and the cross-validation part in part c) but now for different values of. Compare and analyze your results with those obtained in parts a-c). Study the dependence on .
Study also the bias-variance trade-off as function of various values of the parameter. For the bias-variance trade-off, use the bootstrap resampling method. Comment your results.
Part e): Lasso Regression on the Franke function with resampling
This part 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.
Part f): Introducing real data and preparing the data analysis
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 part g) what was done in parts a-e). 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/,
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 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
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.
Part g) OLS, Ridge and Lasso regression with resampling
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).
- 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
- 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 write a report for each project.
- 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.
- 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.
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 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 username 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
- 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
- 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
- sudo apt-get install python3 (or python for python2.7)
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
- 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
- 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
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.