Introductory Statistics Week 1 Lecture slides Introduction –CAST: section 1 –Text: Chapter 1 Exploring Categorical Data: Frequency tables, Pie charts & Bar charts –CAST: section 2.1 –Text: section 2.1 to 2.3

1.2Seven Statistical Stories With Morals Case Study 1.1: Who Are Those Speedy Drivers? Case Study 1.2: Safety in the Skies Case Study 1.3: Did Anyone Ask Whom You’ve Been Dating? Case Study 1.4: Who Are Those Angry Women? Case Study 1.5: Does Prayer Lower Blood Pressure? Case Study 1.6: Does Aspirin Reduce Heart Attack Rates? Case Study 1.7: Does the Internet Increase Loneliness and Depression?

Case Study 1.1 Who Are Those Speedy Drivers? Question: What’s the fastest you have ever driven a car? mph. Data: 87 male and 102 female students from large statistics class at University. Males: Females: Which gender has driven faster? How to summarize data?

Case Study 1.1 Who Are Those Speedy Drivers? Dotplot: each dot represents the response of an individual student.

Case Study 1.1 Who Are Those Speedy Drivers? Five-number summary: the lowest value, the cutoff points for ¼, ½, and ¾ of the data, and the highest value. Note: ¾ of men have driven 95 mph or more, only ¼ of women have done so. Moral:Simple summaries of data can tell an interesting story and are easier to digest than long lists.

Using Minitab Descriptive Statistics: Males, Females Variable Minimum Q1 Median Q3 Maximum Males Females

Importance of Context Focus of statistics –to answer questions that are expressed in the language of some application area Data contain information Statistical methods are used to extract information from data Analysis of data with statistical methods is a core part of statistics, but the context of the data is most important.

Answering a single question in some context

Structure of Data All these 3 data sets have the same basic structure –12 numerical measurements made from 12 different ‘individuals’ –Individuals have been classified into one of two groups Same statistical methods can be applied

Variables and Individuals

Types of variable

Numerical Consists of numerical values taken on each individual (numbers) Discrete –values are whole numbers (counts) –eg. Number of siblings Continuous –any values within some range –eg. Heights Distinction between discrete and continuous variables is important. Statistical methods used for continuous variables are not always appropriate for discrete variables.

Categorical Classifies each individual into one of a small number of categories Ordinal –meaningfully ordered –eg. Tee shirt size: S, M, L, XL grades A, B, C, D, E Nominal –order not meaningful –eg. Eye colour Most statistical methods can be applies to both types of categorical variables.

Labels & Ordering Label variable –each individual may have a unique 'name' that can be used to identify it – May help to identify unusual observations in the data set Individuals in a data set may be ordered. –For example, blood pressure may be recorded from a patient at 10- minute intervals between 9am and 9pm. The resulting blood pressures are a continuous numerical variable whose values are time-ordered - –the ordering of the values holds useful information that will help us understand the data. Unordered data set –the weights of 20 cows sampled from a herd.

Variation Statistics involves measurements (data) in which there is variability – not all measurements are the same. Explained variation –Occasionally the observed variability in a measurement can be explained deterministically in terms of other variables through a law-like relationship. –Example: Ohm’s Law Unexplained variation –In most data sets, some or all variation remains unexplained.

2.1 Raw Data Raw data are for numbers and category labels that have been collected but have not yet been processed in any way. When measurements are taken from a subset of a population, they represent sample data. When all individuals in a population are measured, the measurements represent population data. Descriptive statistics: summary numbers for either population or a sample.

Asking the Right Questions One Categorical Variable Question 1a: How many and what percentage of individuals fall into each category? Example: What percentage of college students favor the legalization of marijuana, and what percentage of college students oppose legalization of marijuana? Question 1b: Are individuals equally divided across categories, or do the percentages across categories follow some other interesting pattern? Example: When individuals are asked to choose a number from 1 to 10, are all numbers equally likely to be chosen?

Asking the Right Questions Two Categorical Variables Question 2a: Is there a relationship between the two variables, so that the category into which individuals fall for one variable seems to depend on which category they are in for the other variable? Example: In Case Study 1.6, we asked if the risk of having a heart attack was different for the physicians who took aspirin than for those who took a placebo. Question 2b: Do some combinations of categories stand out because they provide information that is not found by examining the categories separately? Example: The relationship between smoking and lung cancer was detected, in part, because someone noticed that the combination of being a nonsmoker and having lung cancer is unusual.

Asking the Right Questions One Quantitative Variable Question 3a: What are the interesting summary measures, like the average or the range of values, that help us understand the collection of individuals who were measured? Example: What is the average handspan measurement, and how much variability is there in handspan measurements? Question 3b: Are there individual data values that provide interesting information because they are unique or stand out in some way? Example: What is the oldest recorded age of death for a human? Are there many people who have lived nearly that long, or is the oldest recorded age a unique case?

Asking the Right Questions One Categorical and One Quantitative Variable Question 4a: Are the measurements similar across categories? Example: Do men and women drive at the same “fastest speeds” on average? Question 4b: When the categories have a natural ordering (an ordinal variable), does the measurement variable increase or decrease, on average, in that same order? Example: Do high school dropouts, high school graduates, college dropouts, and college graduates have increasingly higher average incomes?

Asking the Right Questions Two Quantitative Variables Question 5a: If the measurement on one variable is high (or low), does the other one also tend to be high (or low)? Example: Do taller people also tend to have larger handspans? Question 5b: Are there individuals whose combination of data values provides interesting information because that combination is unusual? Example: An individual who has a very low IQ score but can perform complicated arithmetic operations very quickly may shed light on how the brain works. Neither the IQ nor the arithmetic ability may stand out as uniquely low or high, but it is the combination that is interesting.

Explanatory and Response Variables Many questions are about the relationship between two variables. It is useful to identify one variable as the explanatory variable and the other variable as the response variable. In general, the value of the explanatory variable for an individual is thought to partially explain the value of the response variable for that individual.

Summarizing One or Two Categorical Variables First step - count how many fall into each category Frequency Table –Frequency (count) –Relative frequency (proportions or percentage) proportion = frequency in category / total frequency percentage = proportion x 100

Example 2.2 Lighting the Way to Nearsightedness Survey of n = 479 children. Those who slept with nightlight or in fully lit room before age 2 had higher incidence of nearsightedness (myopia) later in childhood. Note: Study does not prove sleeping with light actually caused myopia in more children.

Pie Charts: useful for summarizing a single categorical variable if not too many categories. Bar Graphs: useful for summarizing one or two categorical variables and particularly useful for making comparisons when there are two categorical variables. Visual Summaries for Categorical Variables

Example 2.3 Humans Are Not Good Randomizers Survey of n = 190 college students. “Randomly pick a number between 1 and 10.” Results: Most chose 7, very few chose 1 or 10.

Bar Graphs and Pie Charts Chartjunk –Doesn’t make data easier to understand and can be misleading –Avoid 3D charts –Avoid replacing bars with objects –Better to draw a standard chart smaller than embellishing it with chartjunk Highlight different aspects of the data –Bar charts provides better comparison of the individual proportions –Pie chart allow us to assess the proportions in two or more adjacent categories

Example 2.4 Revisiting Nightlights and Nearsightedness Survey of n = 479 children. Response: Degree of Myopia Explanatory: Amount of Sleeptime Lighting

Stacked Bar Graph