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In the last movie, we looked at how to create scatter plots for two quantitative variables or scale variables in SPSS. Now scatter plots are extremely useful, for exploring new data, and they're also extremely flexible. One variation on this scatter plot though deserves special mention. The time series scatter plot or time plot. As you might guess, the major difference in this case is that the variable that goes across the bottom on the x-axis is some measure of time. Another difference is that time plot often have only one measurement for each time period whereas scatter plots can have, for example, lots of people who are all at the same point on the x-axis.
Because all time plots usually have only one observation at each point in time, you can also connect the points, which makes it more like a line chart. And here's how it works in SPSS. For this example, I'm going to be using the data set that's called NDAQ.sav. And what this is the price for shares in the NASDAQ Exchange itself, from 2002 through 2011. It only has two variables. It has the first market day of each month and it has the closing price on that day for each month.
Let's go up to Graphs and then to Chart Builder and then down to Scatter and choose the Simple Scatter, the top left one, and drag it into the canvas. The Date will go on the bottom and the closing price for the NASDAQ stocks will go on the left, and that's how we need to do right here. I am going just going to click OK, and what you see is a lot of dots. Now, you can see the pattern. It starts relatively low in '04 or '05, shoots way up high in '06 and 'O8, comes back down to earth in 2010, and then starts to go back up again.
But there's a way to make this chart much clearer. We just need to edit it, and do a few different things. So to edit it, like every other chart first we double click on it to open up the editing window. And for this one, what we want to do is we want to click on the button in the menu bar here. It's called Add Interpolation Line. And what this does is it draws a line that connects every dot across the bottom. This is the standard line plot, you would expect every time. Now, if we stop right there, it's not bad.
However, at this point the dots actually get in the way, and so what we can do, is we can click carefully on the dots. So they are all selected and just hit Delete, and we our left with the line plot that shows the pattern more clearly than the dots themselves, of things starting slowly, skyrocketing and then coming back down at the end of the dotcom bubble. And that is a special case where the predictor variable is time and you can adapt the standard scatter plot to show how a variable changes, in which case it's now called the Time Series Scatter Plot or Time Plot.
This is a good example of how SPSS helps you customize your charts to make them easier to read and more useful in interpretation. Up to this point, we have looked at charts for the association of two categorical variables and two scale variables. In the next few movies, we will look at the combination of the two kinds: charts that show the association of one categorical variable and one scale variable.
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