Pearson Product-Moment Correlation is used when you want to measure the strength and direction of the linear relationship between two continuous variables. Here are the key points about when to use it:

1. Variables: Both variables should be continuous (interval or ratio scale).
2. Relationship: You are interested in whether there's a linear association between the two variables.
3. Data distribution: Both variables should be approximately normally distributed.
4. No outliers: The data should not have significant outliers, as these can affect the correlation coefficient.
5. Examples:
   • Relationship between height and weight.
   • Relationship between study time and exam scores.
   • Relationship between temperature and ice cream sales.

If the relationship is non-linear or variables are ordinal/categorical, other methods (like Spearman's rank correlation) might be more appropriate.