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Data Tables
Observations from an experiment are organized into a data table. The Independent Variable is always located on the left side of the data table and the Dependent Variable is on the right.
Data trends are fairly easy to see in simple data tables, like the one shown here. Trends are much harder to see in complicated data tables. Graphs are valuable because they make trends easier to see.
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Variables
The Independent Variable always goes on the horizontal axis of a graph. The experimenter chooses this variable to be the standard by which change is measured during the experiment.
The Dependent Variable always goes on the vertical axis of a graph. This variable changes with changes in the Independent Variable.
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Variable Range
The numeric range of each variable must be calculated before the scale of the axis can be determined. Subtract the lowest data value from the high value. This will give you the range of numbers that must be represented on the axis.
Calculate each variable range seperately. The scale does not have to be the same on each axis.
Notice the range calculations on this data table. The calculations in red produce a range beginning at the lowest data point for each variable. The calculations in blue produce a range beginning at zero.
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Scale
Scale is the number value for each square on an axis. To determine the scale you must know two things:
- The number of squares available along each axis of the graph grid.
- The range of the variable to be represented along each axis.
Divide the largest of these two numbers by the smallest. This will give you the value of each square on the axis.
If the division gives a remainder, round off the number so the data will fit on the graph.
Spread the graph so that data will cover most of the page but NEVER allow the data points to extend past the last square on the graph page.
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Number and Label Each Axis
This grid is being prepared for the data from the table above. For the Independent Variable (horizontal axis), the number of squares (20) is divided by the pH range (2.5). This gives 8 squares per pH. For the Dependent Variable (vertical axis), the range of tadpoles (65) is divided by the number of squares (30). This gives a number just over 2 - so we round up to 3 tadpoles per square.
Using the scale for each axis, write numbers along the axis - increasing from left to right and bottom to top. Notice that neither scale begins with zero. The lowest data points are the beginning of the scale.
Title each axis with the name and units of the variable.
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Plot the Data
Locate each data point with a small dot on the graph.
Place the Dependent Variable data value by each dot - as long as it does not clutter the graph. If the data points are very close together or the values interfere with the graph line, do not add the values to the graph.
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Draw the Line of the Graph
Draw the line or curve that best fits the data points.
Most scientific graphs are not "connect-the-dot" graphs. The purpose of the graph line is to show the general trend of the data. The line does not necessarily have to touch every data point.
Notice how the line drawn here shows the "average trend" of the data.
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Title the Graph
A graph title should clearly tell what the graph is about. This graph shows the title in two locations. While the top location may be the most likely choice, any open space inside the grid may be used - as long as it does not interfere with information on the graph. DO NOT put the title of a graph in the margin of the paper.
If a graph has more than one set of data, a "key" must be included to identify the different lines. Like the title, a graph key should be placed in an open space inside the grid - not in the margin of the paper. Since this graph only has one set of data, a key is not needed.
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Interpreting Graphs
Remember that a scientific graph is usually a straight line or a curved line - not "connect-the-dots". Each type of line represents a certain relationship between the two variables.
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- Straight lines - indicate a direct relationship between the two variables. Every time the Independent Variable changes by a certain amount, the Dependent Variable changes by a certain amount.
In this graph, for every minute of time the distance increases by 0.6 miles. As you know, this indicates that car #1 is moving at a constant speed.
- Curved lines - indicate a changing relationship between the two variables. Every time the Independent Variable changes by a certain amount, the Dependent Variable changes by different amounts.
In this graph, the distance increases more and more for every minute of time. As you know, this indicates that car #1 is now accelerating.
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