Graphs

Graphs

Making and Using Graphs
After scientists organize data in tables, they often manipulate and organize and then display the data in graphs. A graph is a diagram that shows a comparison between variables. Since graphs show a picture of collected data, they make interpretation and analysis of the data easier. The three basic types of graphs used in science are the line graph, bar graph, and pie graph.

A line graph is used to show the relationship between two variables. The variables being compared go on two axes of the graph. The independent variable always goes on the horizontal axis, called the x-axis. The independent variable such as temperature is the condition that is manipulated. The dependent variable always goes on the vertical axis, the y-axis. The dependent variable such as growth is any change that results from manipulating the independent variable.

Suppose a school started a peer-study program with a class of students to see how it affected their science grades.










You could make a graph of the grades of students in the program over a period of time. The grading period is the independent variable and should be placed on the x-axis of your graph. Instead of four grading periods, we could look at average grades for the week or month or year. In this way, we would be manipulating the independent variable. The average grade of the students in the program is the dependent variable and would go on the y-axis.













Plain or graph paper can be used to construct graphs. After drawing your axes, you would label each axis with a scale. The x-axis simply lists the grading periods. To make a scale of grades on the y-axis, you must look at the data values provided in the data table above. Since the lowest grade was 81 and the highest was 89, you know that you will have to start numbering at least at 81 and go through 89. You decide to start numbering at 80 and number by twos spaced at equal distances through 90.

You next must plot the data points. The first pair of data you want to plot is the first grading period and 81. Locate "First" on the x-axis and 81 on the y-axis. Where an imaginary vertical line from the x-axis and an imaginary horizontal line from the y-axis would meet, place the first data point. Place the other data points the same way. After all the points are plotted, connect them with a smooth line.










What if you wanted to compare the average grades of the class in the study group with the grades of another class? The data of the other class can be plotted on the same graph to make the comparison. You must include a key with two different lines, each indicating a different set of data.








Bar graphs are similar to line graphs, except they are used to show comparisons among data or to display data that does not continuously change. In a bar graph, thick bars rather than data points show the relationships among data.

To make a bar graph, set up the x-axis and y-axis as you did for the line graph. The data are plotted by drawing thick bars from the x-axis up to an imaginary point where the y-axis would intersect the bar if it were extended.

Look at the bar graph above comparing the wing vibration rates for different insects. The independent variable is the type of insect, and the dependent variable is the number of wing vibrations per second. The number of wing vibrations for different insects is being compared.








A pie graph uses a circle divided into sections to display data. Each section represents a part of the whole. When all the sections are placed together, they equal 100 percent of the whole.

Suppose you wanted to make a pie graph to show the number of seeds that germinate in a package. You would have to determine the total number of seeds and the number of seeds that germinate out of the total. You count the seeds and find that the package contains 143 seeds. Therefore, the whole pie will represent this amount.

You plant the seeds and determine that 129 seeds germinate. The group of seeds that germinated will make up one section of the pie graph, and the group of seeds that did not germinate will make up another section.

To find out how much of the pie each section should take, you must divide the number of seeds in each section by the total number of seeds. You then multiply your answer by 360, the number of degrees in a circle. Round your answer to the nearest whole number. The number of seeds that germinated would be determined as follows:

143 X 360 = 324.75 or 325
129

* To plot these data on the pie graph, you need a compass and a protractor. Use the compass to draw a circle. Then, draw a straight line from the center to the edge of the circle. Place your protractor on this line, and use it to mark a point on the edge of the circle at 325̊. Connect this point with a straight line to the center of the circle. This is the section for the group of seeds that germinated. The other section represents the group of seeds that did not germinate. Complete the graph by labeling the sections of your graph and giving the graph a title.

Examples
 

1. Create a bar graph to represent the data Draw a conclusion about the relationship between life-span and size of animal based on the data.

 


Practice 2

SKILL BUILDER { MAKING A GRAPH)

WHAT KIND OF GRAPH

WHEN YOU MAKE A GRAPH, THE FIRST STEP IS TO DETERMINE WHAT KIND OF GRAPH TO USE. WHAT YOU WANT TO SHOW AND THE KIND OF DATA YOU HAVE DETERMINE WHICH GRAPH IS THE MOST USEFUL. A PIE GRAPH IS USEFUL FOR SHOWING PARTS IN PROPORTIONS OF A WHOLE. A BAR GRAPH IS USEFUL FOR COMPARING QUANTITIES AND CHANGE OVER TIME. A LINE GRAPH IS GOOD FOR COMPARING TO SET OF DATA OR FOR SHOWING CHANGES AND TRENDS OVER TIME. STUDY THE DATA TABLE BELOW BEFORE ANSWERING THE FOLLOWING QUESTIONS.
 

GROWTH DATA

 

1. WHAT KIND OF GRAPH WOULD YOU USE FOR THE DATA TABLE SHOWING THE CONCRETE COMPOSITION? EXPLAIN YOUR CHOICE.

2. WHAT KIND OF GRAPH WOULD YOU USE FOR THE DATA TABLE SHOWING THE GROWTH DATA? EXPLAIN YOUR CHOICE. WHO IS TALLER AT AGE 8? AT AGE 14?

3. WHAT KIND OF GRAPH WOULD YOU USE FOR THE DATA TABLE SHOWING THE U.S. POPULATION CHANGES SINCE 1910? EXPLAIN YOUR CHOICE.

4. MAKE A GRAPH FOR EACH DATA TABLE SHOWN. FOR EACH GRAPH USE LABELS, SCALES AND TITLES AS NEED