Are you trying to measure a relationship between variables or are you trying to predict variable(s) using one or more other variables?
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More Information (if you need help deciding)
Relationship: For example, what is the relationship between exercising and weight loss? Do they trend in the same direction? Is it a significant relationship?
Prediction: For example, can final exam scores be predicted using time spent studying. How well can this prediction be made? Is the prediction significant? Prediction questions also relate to the specific number-to-number relationship between variables. For instance, how many additional hours of studying are required for an increase in grade on average?