Graphs, Good and Bad

Two types of variables Categorical variables- places individuals into one of categories or groups Examples: gender, ethnic groups, model of car, etc Quantitative Variables – take numerical values for which arithmetic operations such as adding and averaging make sense. Examples: Age, weight, income etc.

Starting with a Table Note- Very important title The Number of Kids per household of parents 25-35 years old Number of households Percent 0 children 8481 15.6 1 child 17071 31.4 2 children 23268 42.8 3 children 4621 8.5 4 plus children 924 1.7 Total 54365 100

Graphs used to organize data Pie charts – used when looking at a fraction of a whole – equals 100%- qualitative or categorical data Bar Graphs- used when data points are few. Used with qualitative data /DV-numerical/IV non-numerical Line Graph- used when you have 2-3 variables, experimental results-quantitative data /IV and DV are both numerical

Pie Charts – use when working with percents Used for Categorical or qualitative Variables – looking at a fraction of a whole To make a pie chart, first draw a circle. The circle represent a whole. Wedges within the circle represent the parts. The angle spanned by each wedge is in proportion to the size of that part. For instance, If we were displaying the amount of education people have had and 21.3% had a bachelor’s degree we would make that slice 77 wide. 0.213  360 = 77

Example – Pie chart Pie Chart – showing the number of children found per household

Bar Graphs Use for Categorical or qualitative Variables The height of the bars show the percent of each category The width of the bars need to be the same

Example - Bar Graphs Bar Graph- showing number of children per household Number of houses Number of children

Pie chart Only compare parts of a whole are often hard to read since it is difficult to compare the angles of the different pieces of the pie Bar graphs can compare quantities that are not parts of a whole Easier to interpret than a pie chart

Line Graph- shows trends, dramatic changes or patterns A line graph is used for graphing quantitative variables that change over time Ex: bank account over time, water levels over time, and performance of stocks Put time (IV) on the horizontal scale( x axis) and the variable you are measuring (DV) on the vertical scale ( y axis) Use with many data points, May have more than 1 variable (line) Connect the data points by lines to display the change over time

Example - Line Graph

Making good graphs Label the x and y axes Tell which units you used for your variables X axis is independent variable/ Y axis is dependent variable Title of Graph – dependent variable vs independent variable