A correlation coefficient is a statistical measure that describes the strength and direction of a relationship between two variables. It ranges from -1 to 1, where -1 indicates a perfect negative correlation, 0 means no correlation, and 1 signifies a perfect positive correlation. For example, if the correlation coefficient between study time and test scores is 0.8, it suggests that as study time increases, test scores tend to increase as well. Correlation coefficients can be calculated using various methods, with the most common being the Pearson correlation coefficient. This method assumes a linear relationship between the variables. Other types include the Spearman rank correlation for non-parametric data and the Kendall tau for ordinal data. Understanding correlation coefficients helps in analyzing data trends and making informed decisions based on relationships between variables.