In the U.S. in 1985, 19,893 people were murdered, compared to 16,848 in 1970 - nearly a 20% increase. ``These figures show that the U.S. became a more violent society over the period 1970-1985.'' True or false, and explain briefly. (from Freedman, et. al, 2nd edition)

Example:

One of the leading causes of death in the U.S. is coronary artery disease, in which the main arteries to the heart break down. This disease can be treated with coronary bypass surgery. In one of the first trials of the operation, Dr. Daniel Ullyot and associates performed coronary bypass surgery on a test group of patients; 98% survived 3 years or more. Previous studies showed that only 68% of the patients getting conventional treatment survived 3 years or more. (The conventional treatment used drugs and special diets to reduce blood pressure and eliminate fatty deposits in the arteries.) A newpaper article described Ullyot's results as ``spectacular''. Comment briefly. (from Freedman, et. al, 2nd edition)

Types of variables

variable: characteristic which changes from person to person in a study

We tend to first classify variables as qualitative or quantitative

A qualitative variable measures levels/categories for which it would not make sense to describe the levels/categories according to a mathematical relation.

A quantitative variable has numerical values for which it does make sense to compare and contrast these values using a mathematical relation.

Nominal

• Can be represented by names of categories or by numbers used to represent these groupings
• Within each group, subjects are homogeneous in terms of the variable
• Groupings must be mutually exclusive
• Groupings should be exhaustive - all subjects must fall into one of the categories
• Representations (by name or number) do not assume any ordered relationships between the categories

Ordinal

• Can be represented by names of categories or by numbers used to represent groupings
• Ordinal variables meet the homogeneous, exhaustive, mutually exclusive criteria
• They assume a rational ordering of categories
• Still cannot say that one is twice as high as another or make other such mathematical statements

Discrete

• Have equality of counting units, each increase in the discrete variable is a jump equal in size to the previous
• Adding, subtracting, multiplying, and dividing make sense in the context of discrete variables
• As long as the counting procedure is accurate, the value is exact

Continuous

• Has an unlimited number of possible values between adjacent values
• Adding, subtracting, multiplying, and dividing make sense in this context
• Theoretically, the value can always be obtained with greater precision