Definitions of Statistics, Probability, and Key Terms

The science of statistics deals with the collection, analysis, interpretation, and presentation of data. We see and use data in our everyday lives.

Learning Outcomes

  • Recognize and differentiate between key terms.


Activity

In your classroom, try this exercise. Have class members write down the average time (in hours, to the nearest half-hour) they sleep per night. Your instructor will record the data. Then create a simple graph (called a dot plot) of the data. A dot plot consists of a number line and dots (or points) positioned above the number line. For example, consider the following data:

55
5.55.5
;
66
66
66
6.56.5
6.56.5
6.56.5
6.56.5
77
77
88
88
99


The dot plot for this data would be as follows:

This is a dot plot showing average hours of sleep. The number line is marked in intervals of 1 from 5 to 9. Dots above the line show 1 person reporting 5 hours, 1 with 5.5, 3 with 6, 4 with 6.5, 2 with 7, 2 with 8, and 1 with 9 hours. Does your dot plot look the same as or different from the example? Why? If you did the same example in an English class with the same number of students, do you think the results would be the same? Why or why not?

Where do your data appear to cluster? How might you interpret the clustering?

The questions above ask you to analyze and interpret your data. With this example, you have begun your study of statistics.

In this course, you will learn how to organize and summarize data. Organizing and summarizing data is called descriptive statistics. Two ways to summarize data are by graphing and by using numbers (for example, finding an average). After you have studied probability and probability distributions, you will use formal methods for drawing conclusions from "good" data. The formal methods are called inferential statistics. Statistical inference uses probability to determine how confident we can be that our conclusions are correct.

Effective interpretation of data (inference) is based on good procedures for producing data and thoughtful examination of the data. You will encounter what will seem to be too many mathematical formulas for interpreting data. The goal of statistics is not to perform numerous calculations using the formulas, but to gain an understanding of your data. The calculations can be done using a calculator or a computer. The understanding must come from you. If you can thoroughly grasp the basics of statistics, you can be more confident in the decisions you make in life.

Probability

Probability is a mathematical tool used to study randomness. It deals with the chance (the likelihood) of an event occurring. For example, if you toss a fair coin four times, the outcomes may not be two heads and two tails. However, if you toss the same coin
4,0004,000
times, the outcomes will be close to half heads and half tails. The expected theoretical probability of heads in any one toss is
1212
or
0.50.5
. Even though the outcomes of a few repetitions are uncertain, there is a regular pattern of outcomes when there are many repetitions. After reading about the English statistician Karl Pearson who tossed a coin
24,00024,000
times with a result of
12,01212,012
heads, one of the authors tossed a coin
2,0002,000
times. The results were
996996
heads. The fraction
9962,000\displaystyle\frac{{996}}{{2,000}}
is equal to
0.4980.498
which is very close to
0.50.5
, the expected probability.

The theory of probability began with the study of games of chance such as poker. Predictions take the form of probabilities. To predict the likelihood of an earthquake, of rain, or whether you will get an A in this course, we use probabilities. Doctors use probability to determine the chance of a vaccination causing the disease the vaccination is supposed to prevent. A stockbroker uses probability to determine the rate of return on a client's investments. You might use probability to decide to buy a lottery ticket or not. In your study of statistics, you will use the power of mathematics through probability calculations to analyze and interpret your data.

Key Terms

In statistics, we generally want to study a population. You can think of a population as a collection of persons, things, or objects under study. To study the population, we select a sample. The idea of sampling is to select a portion (or subset) of the larger population and study that portion (the sample) to gain information about the population. Data are the result of sampling from a population.

Because it takes a lot of time and money to examine an entire population, sampling is a very practical technique. If you wished to compute the overall grade point average at your school, it would make sense to select a sample of students who attend the school. The data collected from the sample would be the students' grade point averages. In presidential elections, opinion poll samples of
1,0001,000
2,0002,000
people are taken. The opinion poll is supposed to represent the views of the people in the entire country. Manufacturers of canned carbonated drinks take samples to determine if a
1616
ounce can contains
1616
ounces of carbonated drink.

From the sample data, we can calculate a statistic. A statistic is a number that represents a property of the sample. For example, if we consider one math class to be a sample of the population of all math classes, then the average number of points earned by students in that one math class at the end of the term is an example of a statistic. The statistic is an estimate of a population parameter. A parameter is a number that is a property of the population. Since we considered all math classes to be the population, then the average number of points earned per student over all the math classes is an example of a parameter.

One of the main concerns in the field of statistics is how accurately a statistic estimates a parameter. The accuracy really depends on how well the sample represents the population. The sample must contain the characteristics of the population in order to be a representative sample. We are interested in both the sample statistic and the population parameter in inferential statistics. In a later chapter, we will use the sample statistic to test the validity of the established population parameter.

A variable, notated by capital letters such as
XX
and
YY
, is a characteristic of interest for each person or thing in a population. Variables may be numerical or categorical. Numerical variables take on values with equal units such as weight in pounds and time in hours. Categorical variables place the person or thing into a category. If we let
XX
equal the number of points earned by one math student at the end of a term, then
XX
is a numerical variable. If we let
YY
be a person's party affiliation, then some examples of
YY
include Republican, Democrat, and Independent.
YY
is a categorical variable. We could do some math with values of
XX
(calculate the average number of points earned, for example), but it makes no sense to do math with values of
YY
(calculating an average party affiliation makes no sense).

Data are the actual values of the variable. They may be numbers or they may be words. Datum is a single value.

Two words that come up often in statistics are mean and proportion. If you were to take three exams in your math classes and obtain scores of
8686
,
7575
, and
9292
, you would calculate your mean score by adding the three exam scores and dividing by three (your mean score would be
84.384.3
to one decimal place). If, in your math class, there are
4040
students and
2222
are men and
1818
are women, then the proportion of men students is
2240\displaystyle\frac{{22}}{{40}}
and the proportion of women students is
1840\displaystyle\frac{{18}}{{40}}
. Mean and proportion are discussed in more detail in later chapters.

NOTE

The words "mean" and "average" are often used interchangeably. The substitution of one word for the other is common practice. The technical term is "arithmetic mean," and "average" is technically a center location. However, in practice among non-statisticians, "average" is commonly accepted for "arithmetic mean."

Example

Determine what the key terms refer to in the following study. We want to know the average (mean) amount of money first year college students spend at ABC College on school supplies that do not include books. We randomly survey
100100
first year students at the college. Three of those students spent
$150\$150
,
$200\$200
, and
$225\$225
, respectively.



Try It

Determine what the key terms refer to in the following study. We want to know the average (mean) amount of money spent on school uniforms each year by families with children at Knoll Academy. We randomly survey
100100
families with children in the school. Three of the families spent
$65\$65
,
$75\$75
, and
$95\$95
, respectively.

Example

Determine what the key terms refer to in the following study.

A study was conducted at a local college to analyze the average cumulative GPA’s of students who graduated last year. Fill in the letter of the phrase that best describes each of the items below.

1._____ Population

2._____ Statistic

3._____ Parameter

4._____ Sample

5._____ Variable

6._____ Data

a) all students who attended the college last year

b) the cumulative GPA of one student who graduated from the college last year

c)

3.653.65
,
2.802.80
,
1.501.50
,
3.903.90

d) a group of students who graduated from the college last year, randomly selected

e) the average cumulative GPA of students who graduated from the college last year

f) all students who graduated from the college last year

g) the average cumulative GPA of students in the study who graduated from the college last year



Example

Determine what the key terms refer to in the following study.

As part of a study designed to test the safety of automobiles, the National Transportation Safety Board collected and reviewed data about the effects of an automobile crash on test dummies. Here is the criterion they used:

Speed at which Cars Crashed Location of “drive” (i.e. dummies)
3535
miles/hour
Front Seat
Cars with dummies in the front seats were crashed into a wall at a speed of
3535
miles per hour. We want to know the proportion of dummies in the driver’s seat that would have had head injuries, if they had been actual drivers. We start with a simple random sample of
7575
cars.



Example

Determine what the key terms refer to in the following study.

An insurance company would like to determine the proportion of all medical doctors who have been involved in one or more malpractice lawsuits. The company selects
500500
doctors at random from a professional directory and determines the number in the sample who have been involved in a malpractice lawsuit.



Activity

Do the following exercise collaboratively with up to four people per group. Find a population, a sample, the parameter, the statistic, a variable, and data for the following study: You want to determine the average (mean) number of glasses of milk college students drink per day. Suppose yesterday, in your English class, you asked five students how many glasses of milk they drank the day before. The answers were
11
,
00
,
11
,
33
, and
44
glasses of milk.

Watch the following video for a brief introduction to statistics.



Concept Review

The mathematical theory of statistics is easier to learn when you know the language. This module presents important terms that will be used throughout the text.

Glossary

Average
also called mean; a number that describes the central tendency of the data


Categorical Variable
variables that take on values that are names or labels


Data
a set of observations (a set of possible outcomes); most data can be put into two groups: qualitative (an attribute whose value is indicated by a label) or quantitative (an attribute whose value is indicated by a number). Quantitative data can be separated into two subgroups: discrete and continuous. Data is discrete if it is the result of counting (such as the number of students of a given ethnic group in a class or the number of books on a shelf). Data is continuous if it is the result of measuring (such as distance traveled or weight of luggage)


Numerical Variable
variables that take on values that are indicated by numbers


Parameter
a number that is used to represent a population characteristic and that generally cannot be determined easily


Population
all individuals, objects, or measurements whose properties are being studied


Probability
a number between zero and one, inclusive, that gives the likelihood that a specific event will occur


Proportion
the number of successes divided by the total number in the sample


Representative Sample
a subset of the population that has the same characteristics as the population


Sample
a subset of the population studied


Statistic
a numerical characteristic of the sample; a statistic estimates the corresponding population parameter.


Variable
a characteristic of interest for each person or object in a population


References

The Data and Story Library, http://lib.stat.cmu.edu/DASL/Stories/CrashTestDummies.html (accessed May 1, 2013).

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