(Re)introduction to Statistics Dan Lizotte

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1 (Re)introduction to Statistics Dan Lizotte Statistics The systematic collection and arrangement of numerical facts or data of any kind; (also) the branch of science or mathematics concerned with the analysis and interpretation of numerical data and appropriate ways of gathering such data. [OED] Why statistics? Can tell you if you should be surprised by your data Can help predict what future data will look like Data ## We'll use data on the duration and spacing of eruptions ## of the old faithful geyser ## Data are eruption duration and waiting time to next eruption data ("faithful") # load data str (faithful) # display the internal structure of an R object ## 'data.frame': 272 obs. of 2 variables: ## $ eruptions: num ## $ waiting : num Data summaries A statistic is a the result of applying a function (summary) to the data: statistic <- function(data) E.g. ranks: Min, Quantiles, Median, Mean, Max summary (faithful$eruptions) ## Min. 1st Qu. Median Mean 3rd Qu. Max. ## Roughly, a quantile for a proportion p is a value x for which p of the data are less than or equal to x. The first quartile, median, and third quartile are the quantiles for p = 0.25, p = 0.5, and p = 0.75, respectively. Visual Summary 1: Box Plot boxplot (faithful$eruptions, main="eruption time", horizontal=t) 1

2 Eruption time Visual Summary 1.5: Box Plot, Jitter Plot library('ggplot2');library(gridextra); #boxplot relatives b1<-ggplot(faithful, aes(x="all",y=eruptions)) + labs(x=null) + geom_boxplot() #jitter plot b2<-ggplot(faithful, aes(x="all",y=eruptions)) + labs(x=null) + geom_jitter(position=position_jitter(height=0,width=0.25)) grid.arrange(b1, b2, nrow=1) 2

3 eruptions 3 eruptions All All Visual Summary 2: Histogram ## Construct histogram of eruption times, plot data points on the x axis hist (faithful$eruptions, main="eruption time", xlab="time (minutes)", ylab="count") points (x=faithful$eruptions,y=rep(0,length(faithful$eruptions)), lwd=4, col='blue') 3

4 Eruption time Count Time (minutes) Visual Summary 2.5: Histogram ## Construct different histogram of eruption times ggplot(faithful, aes(x=eruptions)) + labs(y="proportion") + geom_histogram(aes(y =..count../sum(..count 4

5 Proportion eruptions Visual Summary 3: Empirical Cumulative Distribution Function ## Construct ECDF of eruption times, plot data points on the x axis plot(ecdf(faithful$eruptions), main="eruption time", xlab="time (minutes)", ylab="proportion") points (x=faithful$eruptions,y=rep(0,length(faithful$eruptions)), lwd=4, col='blue') 5

6 Eruption time Proportion Time (minutes) Visual Summary 3.5: Empirical Cumulative Distribution Function ## Different picture of ECDF, with jitter plot ggplot(faithful, aes(x=eruptions)) + labs(x="eruption Time",y="Proportion") + stat_ecdf() + geom_jitter(aes(y=0.125),position=position_jitter(width=0,height=0.1)) 6

7 Proportion Eruption Time Replicates Common assumption is that data consists of replicates that are the same. Come from the same population Come from the same process The goal of data analysis is to understand what the data tell us about the population. Randomness We often assume that we can treat items as if they were distributed randomly. That s so random! Result of a coin flip is random Passengers were screened at random random does not mean uniform Mathematical formalism: events and probability Sample Spaces and Events Sample space S is the set of all possible events we might observe. Depends on context. Coin flips: S = {h, t} Eruption times: S = R 0 7

8 (Eruption times, Eruption waits): S = R 0 R 0 An event is a subset of the sample space. Observe heads: {h} Observe eruption for 2 minutes: {2.0} Observe eruption with length between 1 and 2 minutes and wait between 50 and 70 minutes: [1, 2] [50, 70]. Event Probabilities Any event can be assigned a probability between 0 and 1 (inclusive). Pr({h}) = 0.5 Pr([1, 2] [50, 70]) = 0.10 Probability (OED) Math. As a measurable quantity: the extent to which a particular event is likely to occur, or a particular situation be the case, as measured by the relative frequency of occurrence of events of the same kind in the whole course of experience, and expressed by a number between 0 and 1. An event that cannot happen has probability 0; one that is certain to happen has probability 1. Probability is commonly estimated by the ratio of the number of successful cases to the total number of possible cases, derived mathematically using known properties of the distribution of events, or estimated logically by inferential or inductive reasoning (when mathematical concepts may be inapplicable or insufficient). Axioms of probability Pr is a probability function over S iff 1. For all events A, Pr(A) R, Pr(A) 0 2. Pr(S) = 1 3. If A 1, A 2,... are disjoint, then Pr( A i ) = Pr(A i ) i=1 i=1 Interpreting probability: Objectivist view Suppose we observe n replications of an experiment. Let n(a) be the number of times event A was observed lim n n(a) n = Pr(A) This is (loosely) Borel s Law of Large Numbers (The more correct statment of this is coming up in a few slides.) Subjective interpretation is possible as well. ( Bayesian statistics is related to this idea more later.) 8

9 Abstraction of data: Random Variable We often reduce data to numbers. 1 means heads, 0 means tails. A random variable is a mapping from the event space to a number (or vector.) Usually rendered in uppercase italics X is every statistician s favourite, followed closely by Y and Z. Realizations of X are written in lower case, e.g. x 1, x 2,... We will write the set of possible realizations as: X for X, Y for Y, and so on. Distributions of random variables Realizations are observed according to probabilities specified by the distribution of X Can think of X as an infinite supply of data Separate realizations of the same r.v. X are independent and identically distributed (i.i.d.) Formal definition of a random variable requires measure theory, not covered here Probabilities for random variables Random variable X, realization x. What is the probability we see x? Pr(X = x), (if lazy, Pr(x), but don t do this) Subsets of the domain of a random variable correspond to events. Pr(X > 0) probability that I see a realization that is positive. Discrete Random Variables Discrete random variables take values from a countable set Coin flip X X = {0, 1} Number of snowflakes that fall in a day Y Y = {0, 1, 2,...} Probability Mass Function (PMF) For a discrete X, p X (x) gives Pr(X = x). Requirement: x X p X(x) = 1. Note that the sum can have an infinite number of terms. 9

10 Probability Mass Function (PMF) Example X is number of heads in 20 flips of a fair coin X = {0, 1,..., 20} 0.15 p X (x) x Cumulative Distribution Function (CDF) For a discrete X, P X (x) gives Pr(X x). Requirements: P is nondecreasing sup x X P X (x) = 1 Note: P X (b) = x b p X(x) Pr(a < X b) = P X (b) P X (a) Cumulative Distribution Function (CDF) Example X is number of heads in 20 flips of a fair coin 10

11 P X (x) x Continuous random variables Continuous random variables take values in intervals of R Mass M of a star M = (0, ) Oxygen saturation S of blood S = [0, 1] For a continuous r.v. X, Pr(X = x) = 0 for all x. There is no probability mass function. However, Pr(X (a, b)) 0 in general. Probability Density Function (PDF) For continuous X, Pr(X = x) = 0 and PMF does not exist. However, we define the Probability Density Function f X : Pr(a X b) = b a f X(x) dx Requirement: x f X (x) > 0, f X(x) dx = 1 11

12 Probability Density Function (PDF) Example Density x Cumulative Distribution Function (CDF) For a continuous X, F X (x) gives Pr(X x) = Pr(X (, x]). Requirements: F is nondecreasing sup x X F X (x) = 1 Note: F X (x) = x f X(x) dx Pr(x 1 < X x 2 ) = F X (x 2 ) F X (x 1 ) 12

13 Cumulative Distribution Function (CDF) Example Probability x Expectation The expected value of a discrete random variable X is denoted E[X] = x p X (X = x) x X The expected value of a continuous random variable Y is denoted E[Y ] = y f Y (Y = y) dy y Y E[X] is called the mean of X, often denoted µ or µ X. Sample Mean Given a dataset (collection of realizations) x 1, x 2,..., x n of X, the sample mean is: x n = 1 n Given a dataset, x n is a fixed number. We use X n to denote the random variable corresponding to the sample mean computed from a randomly drawn dataset of size n. i x i 13

14 Datasets and sample means Datasets of size n = 15, sample means plotted in red. (Weak) Law of Large Numbers Informally: If n is large, then x n is probably close to µ X. Formally: lim Pr( X n µ x > ε) = 0 n Statistics, Parameters, and Estimation A statistic is any summary of a dataset. (E.g. function applied to a dataset. Xn, sample median.) A statistic is the result of a A parameter is any summary of the distribution of a random variable. (E.g. µ X, median.) A parameter is the result of a function applied to a distribution. Estimation uses a statistic (e.g. Xn ) to estimate a parameter (e.g. µ X ) of the distribution of a random variable. Estimate: value obtained from a specific dataset Estimator: function (e.g. sum, divide by n) used to compute the estimate Estimand: parameter of interest 14

15 Consistency We often use X n to estimate µ X. Law of Large Numbers is one bit of theory that justifies this choice. An estimator is consistent for an estimand if it converges to the estimand in probability. Sampling Distributions Given an estimate, how good is it? The distribution of an estimator is called its sampling distribution. Bias The expected difference between estimator and parameter. If 0, estimator is unbiased. E[ X n µ X ] Sometimes, Xn > µ X, sometimes X n < µ X, but the long run average of these differences will be zero. Variance The expected squared difference between estimator and its mean 15

16 Positive for all interesting estimators. For an unbiased estimator E[( X n E[ X n ]) 2 ] E[( X n µ X ) 2 ] Sometimes, Xn > µ X, sometimes X n < µ X, but the squared differences are all positive and do not cancel out. Central Limit Theorem Informally: The sampling distribution of Xn is approximately normal if n is big enough. More formally, for X with finite variance: where F Xn ( x) x 1 e ( x µ X ) 2 2σ n 2 σ n 2π σ 2 n = σ2 n is called the standard error and σ 2 is the variance of X. NOTE: More data means lower standard error. Normal (Gaussian) Distribution f X (x) = 1 e (x µ X ) 2 2σ 2 X σ X 2π The normal distribution is special (among other reasons) because many estimators have approximately normal sampling distributions or have sampling distributions that are closely related to the normal. Reminder, σ 2 X = E[(X µ X) 2 ]. If X is normal and we let we have Z = X µ X σ X f Z (z) = 1 2π e z2 2 Who cares? Eruptions dataset has n = 272 observations. Our estimate of the mean of eruption times is x 272 = What is the probability of observing an x 272 that is within 10 seconds of the true mean? 16

17 Who cares? Let σ X272 = σ X / 272, let Z = X 272 µ X σ X272 be a new r.v. By the C.L.T., Pr( 0.17 X 272 µ X 0.17) = Pr( 0.17 σ X272 Z 0.17 σ X272 ) 0.17 σ X272 1 z= 0.17 σ X272 Note! I estimated σ X here. (Look up t-test. ) 2π e z2 2 = z= π e z2 2 = z= π e z2 2 = Density z Confidence Intervals Typically, we specify confidence given by 1 α Use the sampling distribution to get an interval that traps the parameter (estimand) with probability 1 α. 95% C.I. for eruption mean is (3.35, 3.62) 17

18 95% Confidence Region Density z 18

19 What a Confidence Interval Means 19

20 Effect of n on width The Bootstrap CLT gives theoretical approximate sampling distribution of X n. We could also estimate the sampling distribution of Xn by drawing many datasets of size n, computing X n on each, constructing histogram. This is impossible. But we can use the data we have as a surrogate. The Bootstrap Call our dataset D. Draw B new datasets by sampling observations with replacement from D. (B is often at least 1000) Compute X (b) n for each of the datasets. Use the histogram/empirical distribution of these pretend X to determine confidence limits. Bootstrap example library(boot) bootstraps <- boot(faithful$eruptions,function(d,i){mean(d[i])},r=5000) bootdata = data.frame(xbars=bootstraps$t); limits = quantile(bootdata$xbars,c(0.025,0.975)) 20

21 ggplot(bootdata, aes(x=xbars)) + labs(y="prop.") + geom_histogram(aes(y =..density..)) + geom_errorbarh(aes(xmin=limits[[1]], xmax=limits[[2]], y=c(0)),height=0.25,colour="red",size=2) 6 4 Prop xbars 21

22 Reality Check Prop eruptions How much data do I need? Performance measurement: Preview My classifier is correct 20 times out of 30 on this test set! Let X be r.v. representing correctness as {0, 1}. What does µ X mean? Have 50 observations of X. Performance measurement: Preview My classifier is correct 20 times out of 30 on this test set! Let X be r.v. representing correctness as {0, 1}. What does µ X mean? Have 50 observations of X. binom.test(20,30) ## ## Exact binomial test 22

23 ## ## data: 20 and 30 ## number of successes = 20, number of trials = 30, p-value = ## alternative hypothesis: true probability of success is not equal to 0.5 ## 95 percent confidence interval: ## ## sample estimates: ## probability of success ## Test set sample size calculation. Suppose true accuracy is Can I tell the difference from 0.5 with a sample size of 30? How much data would I need to distinguish my classifier from 0.5 with probability (1 β) = 0.8 at a significance level of α = 0.05? Test set sample size calculation. Suppose true accuracy is Can I tell the difference from 0.5 with a sample size of 30? How much data would I need to distinguish my classifier from 0.5 with probability (1 β) = 0.8 at a significance level of α = 0.05? library(pwr) pwr.p.test(h = ES.h(p1 = 0.5, p2 = 0.66), n = NULL, power = 0.8, sig.level = 0.05) ## ## proportion power calculation for binomial distribution (arcsine transformation) ## ## h = ## n = ## sig.level = 0.05 ## power = 0.8 ## alternative = two.sided R summary commands str() shows the structure of a vector, matrix, table, data frame, etc. summary() shows basic summary statistics foo$bar extracts column bar from data frame foo length(),nrow(),ncol() size information for vector, data frame, etc. min(), max(), median(), IQR(), quantile(data,prob) do what you expect IQR is Inter-Quartile Range: 3rd Quartile minus 1st Quartile. mean(), var(), sd() Note: variance and standard deviation use n 1 in denominator Utility commands rep(e,n) creates vector by repeating element e, n times 23

24 which(b) returns list of indices for which boolean expression b is true R commands hist() computes and plots a histogram (probability=t shows proportions instead of frequency) ecdf() boxplot() draws box plot. Whiskers extend at most 1.5 IQR from the nearest quartile. density() constructs a kernel density estimate using given data plot() creates a new scatterplot of given x, y coordinates. Can also be used to plot many other R objects. Try it! points() adds additional points to an existing plot Common function arguments main - Plot title xlab - x label for plot ylab - y label for plot pch - plotting character, what shape to use for points cex - character expansion - multiplicative factor to enlarge/shrink points ggplot2 Very stylish Learning curve steep but worth it Examples in this document Lots of resources on the web 24