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Line Charts and Time-Series Diagrams

Line Charts and Time-Series Diagrams

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Author: Ryan Backman
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Identify changes in quantitative data with line charts and time-series diagram.

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Tutorial

Line Charts

Source: Graphs and Charts created by Author

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Hi. This tutorial covers a common type of graph called the line chart. Actually, what we're going to do first is start with another type of graph called a histogram. So the following histogram shows the average pupil to teacher ratio in the 26 states that are east of the Mississippi River.

So the United States is broken up into to the east of the river and to the west of the river. OK, so let's start by just taking a look at the histogram here, and make sure we understand what it means. So in this case-- so this histogram-- the smallest value is in this 12 to less than 13 range-- so the only number from 12 to less than 13, there end up being-- it's probably 12, so somewhere in that range. And it had a frequency of 2, so that means two states had that ratio.

We can see, from 15 to 16, that was the most frequent. That went up to a frequency of 11, and so forth. So that gives us a pretty good idea of the student-teacher ratio-- or pupil-teacher ratio-- in those states. We'll come back to that in a second.

Now, a line chart is a type of graph where quantitative data is put in the intervals, then dots are plotted to a height determined by the interval frequency. The dots are then connected. Another type of line chart is what's called a frequency polygon. So it's a type of line chart where the dots are placed the tops of the midpoints of bars from a histogram, then the dots are connected.

So they're really very similar. A frequency polygon, you're generally using an existing histogram, but a line chart is basically what's left when you take away the histogram. All right, so let's actually make what'd be considered a frequency polygon in this case. But again, just a type of line chart.

So the first thing we needed to do-- so notice it's already grouped into intervals. A histogram is made. Now what we need to do is we need to use the midpoints of the tops of the histogram bars. So I'm going to start by drawing in these midpoints.

All right, and then what I want to do is connect them using a line. OK, so now, a couple things that this tells us-- it really gives you the same, or very similar information to the histogram. We can tell, when the line flattens out, that's when two categories next to each other have the same frequency. We can see this big spike. Means that this middle category is very different in terms of frequency than the other categories next to it. So this just gives you just another way of representing this quantitative data set.

OK, another type of line chart that's used is what's called a multiple line chart. And a multiple line chart is a type a line chart that can be used to compare quantitative data sets from multiple populations. So you can see, on here, now there's two line graphs-- one in red, the other one in yellow.

The red represents states-- so we're graphing the same information here, average pupil to teacher ratio. The red represents states east of the Mississippi River. That's really the same graph that we saw in the last one. And the yellow represents states west of the Mississippi River. So this bar here represents states west of the Mississippi.

So in this diagram, all 50 states are accounted for. And a couple interesting things you can get from this is it seems that states west of the Mississippi River-- first of all, there's a few states here with a much higher pupil to teacher ratio than any state in the east. We can see, certainly, that the yellow states-- or the states to the west, represented by the yellow line-- don't seem to have this extreme peak to it.

The peak here is at 6, which was in the 14 to 15 range. But this, notice-- when we compare multiple populations on this multiple line chart, we can make some pretty good comparisons of the two separate populations.

So that has been the tutorial on the type of graph known as the line chart Thanks for watching.

Time-Series Diagrams

Source: OREGON PARTY GRAPH, CREATIVE COMMONS: HTTP://EN.WIKIPEDIA.ORG/WIKI/FILE:2001-2009_OREGON_PARTY_COMPARISON.JPG

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Hi, this tutorial covers a type of graph called the time-series diagram. So below is an example of a time-series diagram. So if we take a look at this graph, notice that it's basically like a line chart. It has multiple populations. And this is measuring the 2001-2009 Oregon party comparison. So we're comparing political parties here.

These values are frequencies in number of people starting at from 0, 100,000, 200,000. The three lines here, they're in grayscale. But the top one matches up with the Democrats. Two is the Oregon Republican party.

NAV stands for a non-affiliated voter. That's in the middle here. And then independent party is down here.

OK, now we'll come back to this graph in a sec. But the first thing that-- let's take a look at a definition of what a time-series diagram is. So it's a type of line chart where the x-axis always represents time. So you have to have time on the x-axis.

So time-series diagrams help show changes in a value overtime time. Time-series diagrams are often helpful when comparing populations, whereas you couldn't do the same thing with histograms. You would need to use line charts in order to make a time-series diagram.

So if we just go back to the time-series diagram here, again, we can see that the x-axis is in terms of time, in this case, in the number of years. We can easily compare the populations, as well as seeing how each population changes over time. OK, so we can see that not only has the democratic party seemed to increase in frequency from 2001 to 2009, but it is significant-- it has significantly more members or voters than Republican, non-affiliated voters, or the independent party.

We can see that the Republican Party has really mimicked the decline and growth of the democratic party. But we can see that in the last-- at least since 2008, the democratic party definitely went up quite a bit. So this has been your tutorial on the times-series diagram. Thanks for watching.

Terms to Know
Frequency Polygon

A distribution of data that shows both a histogram and its line chart on the same set of axes.

Line Chart

A distribution of quantitative data that shows the frequency of different intervals of data. The frequencies are indicated by heights of dots, which are connected to each other.

Multiple Line Charts

A distribution that shows more than one data set's values in line charts. This is advantageous because it is clearer than trying to compare multiple histograms on the same set of axes.

Time-Series Diagram

A graphical display that shows the values a variable takes over time.