Ch. 3 Histograms

• In a histogram, the areas of the blocks represent percentages.– Example: Ex. Set B #3

• Types of variables:– Qualitative vs. quantitative

• Discrete vs. continuous

– Example: Ex. Set D #1

• Cross-tabulation– Example: Ex Set F #1

• Any questions from HW?

• Categorical/qualitative variables: Values are groups and represent qualities or order– Examples: Smoking category (yes/no)

Marital status (married, single, divorced, widowed)

Year of college (1st year, sophomore, junior,

senior, FP)

• Quantitative variables: Values that represent an amount. Can be discrete or continuous– Examples: Weight

Number of cigarettes smoke/day

Number of hours spent studying/day

Red blood cell count

• We use bar charts to represent categorical variables

• We can use stem-&-leaf plots to represent quantitative variables

Activity: Martin vs. Westvaco

• Background

• How to read the data

• Bar chart of categorical variables– Bar chart of pay categories by # of employees

• Stem & leaf plot– Plot of all ages

• Do Part I of activity

• How to divide ages (40 or 50 as division)

• Histograms are only used for quantitative variables that are continuous.– Choose the width & endpoints of the intervals– Each bar has an area which represents a %– The bars touch– The height of each bar represents density

• Histogram for age of laid off employees by 10 year intervals– Table (age int., count, percent, percent/year)– Draw the histogram

• What percent of 53 year olds were laid off?

• What percent of 53-55 year olds were laid off?

BEWARE

• Usually all intervals are equal, but you must watch out for those that are not created equally. (Ch. 3 Review Exs. #2, 4)