Which of the following represents a correlation between two variables?

A) A cause-and-effect linkage between two factors
B) The sum of two items in a group
C) The relationship between subsets of data for each item
D) A simultaneous change in both variables
E) A definite pattern over time

A correlation represents a simultaneous change in both variables. A cause-and-effect linkage suggests that one variable causes the other, which is not necessarily the case in a correlation. The sum of two items in a group and the relationship between subsets of data for each item do not necessarily imply a correlation. A definite pattern over time may suggest a trend or seasonality, but not necessarily a correlation between two variables.