STAT 201 Chapter 10: Graphs, Good and Bad

What chart makes it easier for us to compare categories? What are some advantages of this type of chart?

Bar graphs -easy to draw -natural way to order categories -can visually compare different categories, even those not positionally next to each other.

What three things do we look for when studying line graphs?

When studying a line graph, we look for overall patterns/trends, deviations from that pattern, and seasonal variation.

What type of graph do we use to show how quantitative variables change over time?

A line graph shows how quantitative variables change over time.

What is the distribution of a variable?

The distribution of a variable tells us what values it takes and how often it takes those values.

What are some of the disadvantages of using a pie chart? What about pie charts makes them hard for people to visually read?

-hard to draw by hand, -have no natural way to order them -can be hard to compare the sizes of different categories -harder to visually compare angles (how pie charts are drawn) than lengths -hard for people to visually read to make pie chart easier to read: add the percentages falling into each category next to the wedge representing that category.

Why do we use data tables?

-summarize large amounts of information -show us what is going on with the data overall, instead of what is going on with each individual

When creating a line graph of how price of gas changes over time, what would you plot on the X (horizontal) and Y (vertical) axes?

-time on the X (horizontal) axis -variable you are measuring on the Y (vertical) axis

What are the three principals for making good graphs?

1) labels and legends (tell what variables are plotted, their units, and the source of the data) 2) making sure data stands out (do not use unnecessary grids or background art and make sure the placement of the labels doesn't interfere with reading the data) 3) paying attention to what people will see when they read the graph (be careful with scales and don't use pictograms or 3D effects that will confuse the reader)

What three things are necessary for a clear data table?

1) main heading giving subject and date of data 2) labels within table to identify variables & units they are measured in 3) source of data

When making a data table, is it better to present data as counts (for example, the number of people in a category) or as rates (for example, the percent of people in a category)?

It is better to present the data as rates, because they are more informative to someone reading the data table

A pictogram is a variation of a bar chart. Why is it generally a bad idea to use a pictogram use a pictogram?

Pictograms are misleading. Bar graphs are a better idea because in a bar graph all of the bars are the same width, which means when a person is reading it, they only have to compare the height of the different bars. However, in a pictogram, both the heights and the widths of the picture are different for each category, which makes it difficult for people to see the true difference between the categories

Bar charts and pie charts are most useful for what kind of variables?

categorical variables

What are pie charts used to show? What do the wedges within the pie chart represent?

Pie charts are used to show how a whole is divided into parts. -entire circle = the whole -wedges = parts. -size of the pie chart = what portion of the whole fall into that category.

Seasonal Variation

Seasonal variation is a pattern that repeats itself at known regular intervals of time

Seasonal Adjustment

Seasonally adjustment is when the expected seasonal variation is removed before the data is published Ex: graphing unemployment rates or prices of gas over time

You decide to study the average temperature in Chicago each month for many years. Do you expect a line graph of the data to show seasonal variation? Describe the kind of seasonal variation you expect to see

Yes, you would expect a line graph to show seasonal variation. You would expect the average temperatures to be lowest in the winter and highest in the summer, so you expect to see average temperatures to increase during the first half of the year and decrease during the second half of the year.

What type of error do we commonly see in data tables?

roundoff error