positive relationship

negative relationship

no relationship

Positive correlation

Negative correlation

No correlation

Constant correlation

Positive Correlation

Negative Correlation

No Correlation

$$y=-0.83x+0.61$$

$$y=-0.83x+0.63$$

$$y=0.83x+-0.8$$

y = -0.828767x + 0.609589

As the cost goes down, the number sold goes down. 

As the cost goes up, the number sold goes down. 

As the cost does down, the number sold remains the same. 

There is no relationship between cost and the number sold.