Graphs are vital tools for analyzing and displaying data. Graphs allow us to explore the relationship between two quantities -- an **independent variable usually plotted on the x axis and a dependent variable usually plotted on the y-axis. Consider an **experiment measuring the pressure of a gas while its temperature is varied. The **measured** pressure is the** dependent variable**, depending on the temperature set by the experimenter. In plotting a graph by hand, the key is to find a way to plot the two variables so that a straight line is obtained. If you use a computer, on the other hand, software can be used to fit data with a variety of functions. One purpose of this experiment is to compare how you graph by hand to how you can use a computer to help you graph. In general, you will face two types of situations:

1) You have no theory relating the two variables.

2) You have a theory that gives the functional relationship between the variables but the theory contains parameters you need to determine.

In case 1, the relationship between variables can often be determined by investigating different types of graphs. In case 2, the slope and intercept(s) of the graph can be used to determine the values of parameters.