Line Graphs I

Line graphs illustrate the relationship between two major variables. The independent variable is charted on the horizontal axis,
while the dependent is plotted along the vertical axis.

Yearly Instances of Tornadoes in the US, 1950-1999 Source: Storm Prediction Center

The independent variable is the year. It changes no matter what happens to tornadoes. The dependent variable is the total number of tornadoes. It changes depending on the year. Over any 10-year period, it is hard to see a consistent pattern in the data, but over the course of almost 50 years of data, general trends become clearer. What inference can you make about the number of tornadoes occurring in the U.S.? Ans..

Because line graphs display small increases and decreases, they are often considered to be very accurate. These graphs can also handle many different
categories clearly.

Compare the following two charts of the average hourly earnings in selected private industry groups (1982 dollars) from 1970-2000:

Graph A

Graph B Source: U.S. Bureau of Labor Statistics, Employment and Earnings, monthly

In all graphs or charts, it is important to carefully note the unit of measurement used for describing the data:

Accidental Deaths at Home

As is often the case, the independent variable is the year. The dependent variable,accidental deaths, is plotted along the vertical axis. What unit of measurement is being used? How many people died accidentally in their homes in 2000? 29 or 29,000? Be sure to read ALL the labels carefully when analyzing the information involved. There’s a big difference between saying that from 1990 to 2000 the number of accidental deaths increased by 7 rather than 7,000!

You must be especially careful when you write about information where one of the variables used is percentage:

Time Adults Go To Bed Source: Poretz and Sinrod, The First Really Important Survey of American Habits, 1989

In this graph the actual values are included so that you can more easily calculate and analyze the data. The independent variable is the one-hour time period. The dependent variable is percentage of people who indicated that they go to bed during that time. Look at the data for 21-34 year olds. Fifty-one (51%) go to bed between 10 and 11 pm. Only 1% said they go to bed after 12 am. How would you describe that change? Has the number of 21-34 year olds going to bed decreased 50%? NO. A decrease of 50% means the number halved, i.e. dropped to 25% or 26%. But it dropped to 1%. The change is a decrease of 50 percentage points. You can also describe it as a 50-point decrease. Look at the information for 45-55 year olds. Twelve percent go to bed between 11-12 am while 14% go to bed after midnight. That shows an increase of 2 percentage points. A 2% increase would be 12.24, not 14. (By the way, those 2 percentage points equal a 16% increase!) For 35-44 years olds, the percentage going to bed after midnight (29%) shows a larger increase over the percentage going to bed from 11-12 am (14%). Look carefully at the two percentages. How would you describe the change in percentage points? [Ans. An increase of 15 percentage points] In percent? (Hint: 29 is more than double 14.). Ans. Write a statement explaining the data for people 55 and older for the same time period (A1)

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