Math 1 · Unit 10 · Lesson 1 · part 2 of 5

Real data is messy: read the picture first

In Unit 9 every list followed an exact rule: a perfect common difference or a perfect common ratio.

Real data is messy: read the picture first

In Unit 9 every list followed an exact rule: a perfect common difference or a perfect common ratio. Real data is messier. The differences and ratios are only close to equal, and the points only come close to a line or a curve. This unit is about finding the line or exponential curve that fits real data best, and using it to predict. The opening question is the one from Unit 9, Lesson 5: does it look like it adds, or does it look like it multiplies?

Start with the picture. A scatter plot shows two quantities for many cases, one dot per case. The input goes on the x-axis and the output on the y-axis. You met these in 8th grade. Describe any scatter plot three ways.

DirectionGoing up to the right is a positive association: the two quantities go up together. Going down to the right is a negative association: one goes up while the other goes down. No trend at all is no association.

FormDoes the cloud of points look like a line, or like a curve? This is the family question: adds, or multiplies?

StrengthHow tightly do the points hug that shape? Tight is strong. Loosely spread is weak.

The graphing window is a picture of the domain and range. The numbers along the bottom of a scatter plot show roughly the inputs the data covers, its domain. The numbers up the side show roughly the outputs, its range. Before you read any pattern, read the window: what is being measured, and between what values?

Where to go next

Next: One number for direction and strength: rStrength and direction can be packed into one number. All the partsScatter Plots and Correlation