Predicting the probability of achieving the desired results often involves the comparison of two groups. For example, politicians would like to know who is more likely to vote for them, and they will compare the data of potential voters: younger and older, males and females, urban and rural, and so on. Similarly, employees would like to know whether their salary depends more on experience or level of education. Hence, hypothesis testing with two samples may be of great value. In addition, statisticians often make better predictions if they know whether one variable is related to the other. That is, when comparing two variables and attempting to see whether there is a relationship between them, analysts will try to determine whether changes made to one variable trigger corresponding changes in the other variable.

What is the purpose of the scatter diagrams, and how can people determine whether positive, negative, or no correlation exists between two variables by looking at a scatter diagram?