Modern Methods of Data Analysis - SS Introduction

Transcription

1 Modern Methods of Data Analysis Introduction SS 2010 Universität Heidelberg

2 What Do You Learn in this Lecture? How to extract PHYSICS knowledge from measurements in (particle) physics Acquire competence in understanding the statistical tools needed for data analysis understanding the role of uncertainties and probabilities in relating experimental data and theory Beeing able to perform analysis of real data applying these techniques

3 What you NOT learn in this Lecture? How experimental detectors work How the standard model is build How to calculate Feynman diagrams How to design large software projects Mathematical regirous proof of all tools

4 The Particle Physics case... Why would one need to know statistical methods for data analysis in physics? Why should I need to learn such methods? Let's just consider the case of the Standard Model in particle physics...

5 Components of the Standard Model

6 THE STANDARD MODEL!!! Experiments confirm Standard Model to incredible accuracy! Everything is great We found THE THEORY!... is this really all??? Example: Z cross-section: uncertainties smaller than symbol size, data confirm hypothesis: 3 light neutrinos

7 Methodology of Particle Physics Here statistical methods are crucial!

8 How do we reach those goals? Introduction to statistical concepts in the lectures with integrated exercises Hands-on work in the computer exercises optional at end of semester: reproduce physics analysis on real (LHC) data -> presentation Slides of lecture available Monday afternoon before the lecture

9 Computer Exercises: Place: CIP-Pool, Albert-Ueberle-Str. 3-5 Potential dates: Wednesday: h Thursday: h Which programming skills do you have? C++, ROOT, other languages Do you plan to attend computer course? Do you have a Rechenzentrums-account?

10 Credit Points Lecture: 2 CP - requirements: short oral exam at end of the term (can be decided shortly before end of the term) Computer Exercises: 2 CP - requirement: (very) frequent presence

11 Outline of the Lecture Basics Concepts & Definitions Characteristics of distribution Monte Carlo generators Important distributions Error propagation Estimators Maximum Likelihood Least Square method Confidence Level and Limits Hypothesis Tests introduction/bring everyone to same level core part of the lecture Add. Material multivariant systems, analysis bias, numerical methods, function minimization

12 The Cheating Baker Once upon a time, in a holiday resort the landlord L. ran a profitable B&B, and every morning bought 30 rolls for breakfast. By law the mass of a single roll was required to be 75 g. One fine day the owner of the bakery changed, and L. suspected that the new baker B. might be cheating. So he decided to check the mass of what he bought, using a kitchen scales with a resolution of 1g. After one month he had collected a fair amount of data:

13 Data Reduction the raw list of number is not very useful need some kind of data reduction assume that all measurements are equivalent the sequence of individual data does not matter all relevant information is contained in the number of counts per reading - much improved presentation of the collected information - the above numbers cover the entire data set - most of the measurements are lower than 75g...

14 Visualization an even better presentation of the available information bar-chart of the tables given before example for the concept of a histogram define bins for the possible values of a variable plot the number of entries in each bin immediate grasp of center and width of the distribution The rolls produced by baker B. definitely show a lack of doe. So L. was right in his suspicion, that B tried to make some extra profit by cheating...

15 and the Conclusion As a consequence of his findings, L. complained to B. and even threatened to inform the police about his doings. B. apologized and claimed that the low mass of the rolls was an accident which will be corrected in the future. L., however, continues to monitor the quality delivered by the baker. One month later, B. inquired again about the quality of his products, asking whether now everything was all right. L., for his part, acknowledged that the weight of the rolls now matched his expectations, but he also voiced the opinion that B. was still cheating... B. simply selected the heaviest rolls for L.!

16 Literature Statistische und numerische Methoden der Datenanalyse (V. Blobel/E.Lohrmann) Statistical Data Analysis (Glen Cowan) Statistics: A guide to the use of statistical methods in Physical Sciences (R. J. Barlow) Recommendation for a rainy Sunday afternoon:

17 Many good lectures on statistical methods around Lecture SS07 Christoph Grab & Christian Regenfus Uni Zuerich Michael Feindt, Guenther Quast, IEKP Karlsruhe, many different lectures and many good exercise examples! Guenter Duckeck, Madjid Boutemeur Lecture Ian C. Brock, WS 2000/01, Universitaet Bonn Michael Schmelling, Heidelberger Graduiertentag 2006

18 Modern Methods of Data Analysis Lecture I Content: - Basic Concepts of Probability - Definition of random variables, Probability density, mean, variance...

19 Predictable For easy classical physical processes the result is precisely determined determinism Determinisums Examples are : pendular, orbit of planets, billard...

20 Contingency Random events are as a matter of principle not predictable (even with precise knowledge of the starting conditions) Examples: Lotto (depend on many quantities, deterministic chaos) radioactive decay (quantum mechanics) electronic noise measurement uncertainties

21 Quantum Mechanics Each time something different happens... events of the CDF experiment

22 Measurements Experiments measure frequency distributions:

23 Probability vs. Statistics Probability: from theory to data start with a well-defined problem and calculate all possible outcomes of a specific experiment Statistics: from data to theory Try to solve the inverse problem: starting from a data set, want to deduce the rules/laws this is analyzing experimental data deals with parameter estimation determine parameters and uncertainties in unbiased and efficient way hypothesis testing (agreement,confidence ) - most challenging: to match question and answers

24 Definition/Interpretation of Probability mathematical probability (axiomatic) all definitions of probability need to follow these axioms interpretation as rel. Frequency(Frequentists) subjective probability (Bayesian) Beware of different names used in literature! Lucky enough, in all useful cases they use the same combinatorial rules of probabilities! Interpretation of results differ sometimes... Not go in detail about difference at this stage of the lecture.

25 Kolmogorov Axioms (1931) Elementary events are considered, which exclude each other: elementary event set of all elements probability of event positive definite additive normalized Formal approach, however from pragmatic point of view rather useless...

26 Combination of Probabilities : A or B : A and B : not A : P(A) if is reduced to B (conditional bedingte probability) Can be deduced from Kolmogorov axioms: A and B are called independent if:

27 Frequentist Probability empirical definition via frequency of occurrence (R. von Mises) perform experiment N times in identical trials, assume event E occurs k times, then: P(E) = lim k/n intuitive definition for particle physics (many repetitions) very useful, but has some problems limes in strict mathematical sense does not exist, i.e. can not be proven that it converges... how large is N? When does it converge? What means repeatable under identical conditions? Is similar OK as well? This IS difficult, if not impossible... Not applicable for single events.

28 Subjective (Bayesian) Probability Reverend Thomas Bayes ( ) Probability is the degree of believe, that an experiment has a specific result. Subjective probability compatible with Kolmogorov axioms Essay Towards Solving a Problem in the Doctrine of Chances (1763), published postum in Philosophical Transactions of the Royal Society of London

29 Examples for Bayesian Probability Frequentists interpretation often not applicable. Than Bayesian interpretation only possible one. Probability is the degree of belief that a hypothesis is true: - The particle in this event is a positron. - Nature is supersymmetric. - It will rain tomorrow. - Germany will win the soccer championship this year. Often criticized as subjective and non scientific. However based on simple probability computation (axioms). Not contradicting the Frequentists approach, but include it.

30 Comments on Probability There are strong rivaling schools of these approaches Be beware of names: Bayesians call frequentists the classical ones some call the frequency def. of probability objective Most scientist would claim to belong to the frequency school But we often talk in Bayesian probability terms; and we think of probabilites as objective numbers In particular, any attempt of interpreating results of an experiment falls into the trap of repeatability. Bayes' Theorem will highlight this more...

31 Bayes' Theorem (1) Conditional ( bedingte ) probability Due to follows:

32 Examples: Rare Disease (1) Probability of disease A: P(A) = P(not A) = Test for disease: P(+ A) = 0.98 P(- A) = 0.02 P(+ not A) = 0.03 P(- not A) = 0.97 Do you need to be worried if you get + as test result? What is the posteriori probability P(A +)?

33 Example: Rare Disease (2) The posterior probability is only 3.2%, due to the tiny prior probability and the non negligible misidentification rate.

34 Exercise: Particle ID Example particle ID detector: proton-antiproton collision: 90% pions, 10% kaons kaon ID 95% efficient, pion mis-id 6% Question: If the particle ID indicates a kaon, what is the probability that it is a real kaon/a real pion?

35 Bayes' Theorem (2) Important through the interpretation A=theory,B=data Likelihood Posterior Prior Evidence

36 Confirmation and Rejection P(no Higgs below 200 GeV SM is true) < Likelihood If I believe the SM is true, what is the probability to see no Higgs below 200 GeV: no Higgs below 200 GeV SM is ruled out P(SM is true Higgs below 200 GeV exists) depends on P(Higgs below 200 GeV in any model) and on the a priori probability that P(SM is true) Posteriori Probability If I see a Higgs below 200 GeV, how large is the probability that the SM is true?

37 Some Definitions experiment e.g. throwing 2 times a dice realization: outcome of one specific experiment event space/sample space: all possible outcomes random variable Zufallsvariable function of outcome of experiment (e.g. sum of dices) event space and realization defined accordingly for random variables random variable can be either discrete or continues

38 Example experiment: throwing two dices event space of experiment {11,12,13,14,15,16,21,22,23,...,64,65,66} random variable x = sum of dices event space of x: {2,3,4,5,6,7,8,9,10,11,12} realization of experiment e.g. 25 accordingly realization of x=7 cumulated distribution u(x): probability to observe x or smaller value

39 Probability density function (pdf) Repeat an experiment with outcome characterized by single continuous variable x Definition: the probability to measure a value x in the interval [x,x+dx] is give by probability density function f(x) (pdf) Wahrscheinlichkeitsdichte pdf f(x) >=0 and normalized pdf f(x) is NOT a probability, it has dimension 1/x!

40 Question on PDFs List some examples of a probability density function f(x) continuous ones discrete ones

41 Cumulative distribution function (cdf) Cumulative distribution function F(x), also know as probability distribution function. Wahrscheinlichkeitsverteilung = Verteilungsfunktion F(x') is interpreted as probability to find value x <= x' F(x) is continuously non-decreasing function F(- ) = 0 and F(+ )=1 is directly related to the probability density function f(x) by: f(x) is given (for well-behaved distributions) by:

42

43 Example: cdf f(x) = 1-x for x in [0,2] 0 elsewhere Compute the function F(x) What is the probability to find x > 1.5? What is the probability to find x in [0.5,1]

44 Arithmetic Mean Definition: Examples: Average number of children per family in Germany is 2.3 Average lifetime expectation for men is 74 for women 78 Average amount of semester for physics studies in Heidelberg is

45 Weighted Mean Definition: Example: 5 measurements { uncertainties { } with different } arithmetic mean is special case of weighted means (same weight for each measurement)

46 ... even more averages Mode/Modus: most probable value (highest bin in distribution) definition not unique, unimodal, bimodal distributions Median: smallest value which is than 50% of the events (median more robust against outliers than artithm.mean) For unimodal, symmetric functions centered around : median = mode = arithmetic mean else, for unimodal functions, empiric rule mean-mode = 3 x (mean-median)

47 Examples: Give median, mode, arithmetic mean of:

48 Energy loss in Material Assume tracking stations with 12 layers The energy loss per traversed material (de/dx) of the particle traversing a layer follows a Landau distribution mode de/dx

49 Energy loss in Material The mode of the de/dx distribution as a function of particle momentum is used to seperate different particle species 1 10 momentum [GeV/c] How to get estimate of mode from 12 measurements?

50 Truncated Mean Discard 10/20% measurements with larges value (symmetrize the function) Take arithmetic mean of remaining ones All about estimating the true mode/median/mean of distributions from given data set. More about estimators later.